diff --git a/app/models/media_asset.rb b/app/models/media_asset.rb
index 3be6214e5..9ef8598a0 100644
--- a/app/models/media_asset.rb
+++ b/app/models/media_asset.rb
@@ -10,8 +10,11 @@ class MediaAsset < ApplicationRecord
     active: 200,
     deleted: 300,
     expunged: 400,
+    failed: 500,
   }
 
+  validates :md5, uniqueness: { conditions: -> { where(status: [:processing, :active]) } }
+
   class Variant
     attr_reader :media_asset, :variant
     delegate :md5, :storage_service, :backup_storage_service, to: :media_asset
@@ -120,11 +123,46 @@ class MediaAsset < ApplicationRecord
 
   concerning :FileMethods do
     class_methods do
+      # Upload a file to Danbooru. Resize and distribute it then return a MediaAsset.
+      #
+      # If the file has already been uploaded to Danbooru, then return the
+      # existing MediaAsset. If someone else is uploading the same file at the
+      # same time, wait until they're finished and return the existing
+      # MediaAsset. If distributing the file fails, then mark the MediaAsset as
+      # failed and raise an exception.
+      #
+      # This can't be called inside a transaction because the transaction will
+      # fail if there's a RecordNotUnique error when the asset already exists.
       def upload!(media_file)
         media_asset = create!(file: media_file, status: :processing)
         media_asset.distribute_files!(media_file)
         media_asset.update!(status: :active)
         media_asset
+
+      # If the file has already been uploaded, then the `create!` call will raise one of these errors.
+      rescue ActiveRecord::RecordInvalid, ActiveRecord::RecordNotUnique => e
+        raise if e.is_a?(ActiveRecord::RecordInvalid) && !e.record.errors.of_kind?(:md5, :taken)
+
+        media_asset = find_by!(md5: media_file.md5, status: [:processing, :active])
+
+        # XXX If the asset is still being processed by another thread, wait up
+        # to 30 seconds for it to finish.
+        if media_asset.processing?
+          30.times do
+            break if !media_asset.processing?
+            sleep 1
+            media_asset.reload
+          end
+
+          # If the asset is still processing after 30 seconds, or if it moved
+          # from the processing state to the failed state, then fail.
+          raise "Upload failed" if !media_asset.active?
+        end
+
+        media_asset
+      rescue Exception
+        media_asset&.update!(status: :failed)
+        raise
       end
     end
 
diff --git a/app/models/post.rb b/app/models/post.rb
index e32ea6e4a..3ec673ec5 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -859,12 +859,10 @@ class Post < ApplicationRecord
     end
 
     def replace!(params)
-      transaction do
-        replacement = replacements.create(params)
-        processor = UploadService::Replacer.new(post: self, replacement: replacement)
-        processor.process!
-        replacement
-      end
+      replacement = replacements.create(params)
+      processor = UploadService::Replacer.new(post: self, replacement: replacement)
+      processor.process!
+      replacement
     end
   end
 
diff --git a/db/migrate/20211023225730_add_unique_md5_constraint_on_media_assets.rb b/db/migrate/20211023225730_add_unique_md5_constraint_on_media_assets.rb
new file mode 100644
index 000000000..55dbf1a4a
--- /dev/null
+++ b/db/migrate/20211023225730_add_unique_md5_constraint_on_media_assets.rb
@@ -0,0 +1,5 @@
+class AddUniqueMd5ConstraintOnMediaAssets < ActiveRecord::Migration[6.1]
+  def change
+    add_index :media_assets, :md5, name: "index_media_assets_on_md5_and_status", unique: true, where: "status IN (100, 200)"
+  end
+end
diff --git a/db/structure.sql b/db/structure.sql
index 318c286e7..24a78d728 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -3779,6 +3779,13 @@ CREATE INDEX index_media_assets_on_image_width ON public.media_assets USING btre
 CREATE INDEX index_media_assets_on_md5 ON public.media_assets USING btree (md5);
 
 
+--
+-- Name: index_media_assets_on_md5_and_status; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE UNIQUE INDEX index_media_assets_on_md5_and_status ON public.media_assets USING btree (md5) WHERE (status = ANY (ARRAY[100, 200]));
+
+
 --
 -- Name: index_media_assets_on_status; Type: INDEX; Schema: public; Owner: -
 --
@@ -5049,6 +5056,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20211014063943'),
 ('20211015223510'),
 ('20211018045429'),
-('20211018062916');
+('20211018062916'),
+('20211023225730');
 
 
diff --git a/test/factories/post.rb b/test/factories/post.rb
index 931709b12..a7949a5b2 100644
--- a/test/factories/post.rb
+++ b/test/factories/post.rb
@@ -13,5 +13,20 @@ FactoryBot.define do
     rating {"q"}
     source { FFaker::Internet.http_url }
     media_asset { build(:media_asset) }
+
+    factory(:post_with_file) do
+      transient do
+        filename { "test.jpg" }
+        media_file { MediaFile.open("test/files/#{filename}") }
+      end
+
+      md5 { media_file.md5 }
+      image_width { media_file.width }
+      image_height { media_file.height }
+      file_ext { media_file.file_ext }
+      file_size { media_file.file_size }
+
+      before(:create) { |post, evaluator| MediaAsset.upload!(evaluator.media_file) }
+    end
   end
 end
diff --git a/test/unit/post_test.rb b/test/unit/post_test.rb
index a006a04ba..bd75f1814 100644
--- a/test/unit/post_test.rb
+++ b/test/unit/post_test.rb
@@ -31,8 +31,7 @@ class PostTest < ActiveSupport::TestCase
   context "Deletion:" do
     context "Expunging a post" do
       setup do
-        @upload = UploadService.new(FactoryBot.attributes_for(:jpg_upload)).start!
-        @post = @upload.post
+        @post = create(:post_with_file, uploader: @user, filename: "test.jpg")
         Favorite.create!(post: @post, user: @user)
         create(:favorite_group, post_ids: [@post.id])
         perform_enqueued_jobs # perform IqdbAddPostJob
diff --git a/test/unit/upload_service_test.rb b/test/unit/upload_service_test.rb
index e8226b851..73a18406d 100644
--- a/test/unit/upload_service_test.rb
+++ b/test/unit/upload_service_test.rb
@@ -224,31 +224,29 @@ class UploadServiceTest < ActiveSupport::TestCase
         end
       end
 
-      subject { UploadService::Replacer.new(post: @post, replacement: @replacement) }
-
       context "#process!" do
         should "create a comment" do
-          assert_difference(-> { @post.comments.count }) do
-            as(@user) { subject.process! }
-            @post.reload
+          assert_difference(-> { @post.reload.comments.count }) do
+            as(@user) { @post.reload.replace!(replacement_url: "", replacement_file: @new_file) }
           end
         end
 
         should "not create a new post" do
           assert_difference(-> { Post.count }, 0) do
-            as(@user) { subject.process! }
+            as(@user) { @post.reload.replace!(replacement_url: "", replacement_file: @new_file) }
           end
         end
 
         should "update the post's MD5" do
-          assert_changes(-> { @post.md5 }) do
-            as(@user) { subject.process! }
-            @post.reload
+          assert_changes(-> { @post.reload.md5 }) do
+            as(@user) { @post.reload.replace!(replacement_url: "", replacement_file: @new_file) }
           end
         end
 
         should "preserve the old values" do
-          as(@user) { subject.process! }
+          as(@user) { @post.reload.replace!(replacement_url: "", replacement_file: @new_file) }
+          @replacement = @post.replacements.last
+
           assert_equal(1500, @replacement.old_image_width)
           assert_equal(1000, @replacement.old_image_height)
           assert_equal(2000, @replacement.old_file_size)
@@ -257,8 +255,10 @@ class UploadServiceTest < ActiveSupport::TestCase
         end
 
         should "record the new values" do
-          as(@user) { subject.process! }
-          assert_equal(500, @replacement.image_width)
+          as(@user) { @post.reload.replace!(replacement_url: "", replacement_file: @new_file) }
+          @replacement = @post.replacements.last
+
+          assert_equal(500, @replacement.reload.image_width)
           assert_equal(335, @replacement.image_height)
           assert_equal(28086, @replacement.file_size)
           assert_equal("jpg", @replacement.file_ext)
@@ -266,7 +266,9 @@ class UploadServiceTest < ActiveSupport::TestCase
         end
 
         should "correctly update the attributes" do
-          as(@user) { subject.process! }
+          as(@user) { @post.reload.replace!(replacement_url: "", replacement_file: @new_file) }
+          @replacement = @post.replacements.last
+
           assert_equal(500, @post.image_width)
           assert_equal(335, @post.image_height)
           assert_equal(28086, @post.file_size)
@@ -280,7 +282,7 @@ class UploadServiceTest < ActiveSupport::TestCase
         should "raise an error" do
           upload_file("test/files/test.png") do |file|
             assert_raises(UploadService::Replacer::Error) do
-              as(@user) { @post.replace!(replacement_file: file, replacement_url: "") }
+              as(@user) { @post.reload.replace!(replacement_file: file, replacement_url: "") }
             end
           end
         end
@@ -304,14 +306,10 @@ class UploadServiceTest < ActiveSupport::TestCase
         end
       end
 
-      subject { UploadService::Replacer.new(post: @post, replacement: @replacement) }
-
       should "replace the post" do
-        as(@user) { subject.process! }
+        as(@user) { @post.reload.replace!(replacement_url: @new_url) }
 
-        @post.reload
-
-        assert_equal(@new_url, @post.replacements.last.replacement_url)
+        assert_equal(@new_url, @post.reload.replacements.last.replacement_url)
       end
     end
 
@@ -330,12 +328,10 @@ class UploadServiceTest < ActiveSupport::TestCase
         end
       end
 
-      subject { UploadService::Replacer.new(post: @post, replacement: @replacement) }
-
       context "when replacing with its own source" do
         should "raise an error" do
           assert_raises(UploadService::Replacer::Error) do
-            as(@user) { @post.replace!(replacement_url: @post.source) }
+            as(@user) { @post.reload.replace!(replacement_url: @post.source) }
           end
         end
       end
@@ -351,7 +347,7 @@ class UploadServiceTest < ActiveSupport::TestCase
 
         should "throw an error" do
           assert_raises(UploadService::Replacer::Error) do
-            as(@user) { @post2.replace!(replacement_url: @new_url) }
+            as(@user) { @post2.reload.replace!(replacement_url: @new_url) }
           end
         end
       end
@@ -361,7 +357,7 @@ class UploadServiceTest < ActiveSupport::TestCase
           replacement_url = "https://cdn.donmai.us/original/fd/b4/fdb47f79fb8da82e66eeb1d84a1cae8d.jpg"
           final_source = "https://cdn.donmai.us/original/71/0f/710fd9cba4ef37260f9152ffa9d154d8.png"
 
-          as(@user) { @post.replace!(replacement_url: replacement_url, final_source: final_source) }
+          as(@user) { @post.reload.replace!(replacement_url: replacement_url, final_source: final_source) }
 
           assert_equal(final_source, @post.source)
         end
@@ -372,7 +368,7 @@ class UploadServiceTest < ActiveSupport::TestCase
           skip "Twitter key not set" unless Danbooru.config.twitter_api_key
           replacement_url = "https://twitter.com/nounproject/status/540944400767922176"
           image_url = "https://pbs.twimg.com/media/B4HSEP5CUAA4xyu.png:orig"
-          as(@user) { @post.replace!(replacement_url: replacement_url) }
+          as(@user) { @post.reload.replace!(replacement_url: replacement_url) }
 
           assert_equal(replacement_url, @post.replacements.last.replacement_url)
         end
@@ -384,7 +380,8 @@ class UploadServiceTest < ActiveSupport::TestCase
           as(@user) do
             @post = FactoryBot.create(:post, source: "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg")
             @post.stubs(:queue_delete_files)
-            @post.replace!(replacement_url: "https://cdn.donmai.us/original/fd/b4/fdb47f79fb8da82e66eeb1d84a1cae8d.jpg", tags: "-tag1 tag2")
+            @new_url = "https://cdn.donmai.us/original/fd/b4/fdb47f79fb8da82e66eeb1d84a1cae8d.jpg"
+            @post.reload.replace!(replacement_url: @new_url, tags: "-tag1 tag2")
           end
 
           @replacement = @post.replacements.last
@@ -392,7 +389,8 @@ class UploadServiceTest < ActiveSupport::TestCase
 
         should "update the attributes" do
           as(@user) do
-            subject.undo!
+            replacer = UploadService::Replacer.new(post: @post.reload, replacement: @replacement)
+            replacer.undo!
           end
 
           assert_equal("tag2", @post.tag_string)
@@ -408,41 +406,39 @@ class UploadServiceTest < ActiveSupport::TestCase
 
       context "#process!" do
         should "create a comment" do
-          assert_difference(-> { @post.comments.count }) do
-            as(@user) { subject.process! }
-            @post.reload
+          assert_difference(-> { @post.reload.comments.count }) do
+            as(@user) { @post.reload.replace!(replacement_url: @new_url) }
           end
         end
 
         should "not create a new post" do
           assert_difference(-> { Post.count }, 0) do
-            as(@user) { subject.process! }
+            as(@user) { @post.reload.replace!(replacement_url: @new_url) }
           end
         end
 
         should "update the post's MD5" do
-          assert_changes(-> { @post.md5 }) do
-            as(@user) { subject.process! }
-            @post.reload
+          assert_changes(-> { @post.reload.md5 }) do
+            as(@user) { @post.reload.replace!(replacement_url: @new_url) }
           end
         end
 
         should "update the post's source" do
-          assert_changes(-> { @post.source }, nil, from: @post.source, to: @new_url) do
-            as(@user) { subject.process! }
+          assert_changes(-> { @post.reload.source }, nil, from: @post.source, to: @new_url) do
+            as(@user) { @post.reload.replace!(replacement_url: @new_url) }
             @post.reload
           end
         end
 
         should "not change the post status or uploader" do
           assert_no_changes(-> { {ip_addr: @post.uploader_ip_addr.to_s, uploader: @post.uploader_id, pending: @post.is_pending?} }) do
-            as(@user) { subject.process! }
+            as(@user) { @post.reload.replace!(replacement_url: @new_url) }
             @post.reload
           end
         end
 
         should "leave a system comment" do
-          as(@user) { subject.process! }
+          as(@user) { @post.reload.replace!(replacement_url: @new_url) }
           comment = @post.comments.last
           assert_not_nil(comment)
           assert_equal(User.system.id, comment.creator_id)
@@ -457,7 +453,7 @@ class UploadServiceTest < ActiveSupport::TestCase
 
         should "replace with the full size image" do
           as(@user) do
-            @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+            @post.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
           end
 
           assert_equal(80, @post.image_width)
@@ -474,7 +470,7 @@ class UploadServiceTest < ActiveSupport::TestCase
           @post.unstub(:queue_delete_files)
           FileUtils.expects(:rm_f).times(3)
 
-          as(@user) { @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350") }
+          as(@user) { @post.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350") }
 
           travel_to((PostReplacement::DELETION_GRACE_PERIOD + 1).days.from_now) do
             perform_enqueued_jobs
@@ -488,7 +484,7 @@ class UploadServiceTest < ActiveSupport::TestCase
           skip "Pixiv credentials not configured" unless Sources::Strategies::Pixiv.enabled?
 
           begin
-            as(@user) { @post.replace!(replacement_url: "http://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364") }
+            as(@user) { @post.reload.replace!(replacement_url: "http://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364") }
             @post.reload
 
             assert_equal(80, @post.image_width)
@@ -515,12 +511,10 @@ class UploadServiceTest < ActiveSupport::TestCase
           #FileUtils.expects(:rm_f).times(3)
 
           as(@user) do
-            @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
-            @post.reload
-            @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
-            @post.reload
+            @post.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+            @post.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
             Upload.destroy_all
-            @post.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+            @post.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
           end
 
           assert_nothing_raised { @post.file(:original) }
@@ -550,20 +544,17 @@ class UploadServiceTest < ActiveSupport::TestCase
           as(@user) do
             skip unless MediaFile::Ugoira.videos_enabled?
 
-            @post1.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
-            @post2.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
+            @post1.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+            @post2.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
             assert_equal("4ceadc314938bc27f3574053a3e1459a", @post1.md5)
             assert_equal("cad1da177ef309bf40a117c17b8eecf5", @post2.md5)
 
-            @post2.reload
-            @post2.replace!(replacement_url: "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg")
+            @post2.reload.replace!(replacement_url: "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg")
             assert_equal("d34e4cf0a437a5d65f8e82b7bcd02606", @post2.md5)
             Upload.destroy_all
-            @post1.reload
-            @post2.reload
 
-            @post1.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
-            @post2.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
+            @post1.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
+            @post2.reload.replace!(replacement_url: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350")
             assert_equal("cad1da177ef309bf40a117c17b8eecf5", @post1.md5)
             assert_equal("4ceadc314938bc27f3574053a3e1459a", @post2.md5)
           end
@@ -599,7 +590,7 @@ class UploadServiceTest < ActiveSupport::TestCase
             assert_difference(-> { @note.versions.count }) do
               # replacement image is 80x82, so we're downscaling by 50% (160x164 -> 80x82).
               as(@user) do
-                @post.replace!(
+                @post.reload.replace!(
                   replacement_url: "https://i.pximg.net/img-original/img/2017/04/04/08/54/15/62247350_p0.png",
                   final_source: "https://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247350"
                 )
@@ -690,7 +681,8 @@ class UploadServiceTest < ActiveSupport::TestCase
 
       context "when the file has already been uploaded" do
         setup do
-          @post = create(:post, md5: "d34e4cf0a437a5d65f8e82b7bcd02606")
+          @asset = MediaAsset.find_by_md5("d34e4cf0a437a5d65f8e82b7bcd02606")
+          @post = create(:post, md5: "d34e4cf0a437a5d65f8e82b7bcd02606", media_asset: @asset)
           @service = subject.new(source: @source)
         end