Fix #4470: Check URLs for duplicates when creating artists

Show a warning when creating a duplicate artist; that is, when adding a
URL that already belongs to another artist.

This is a soft warning rather than a hard error because there are some
cases where multiple artists legitimately share the same site or account.

test/functional/artists_controller_test.rb

@@ -197,6 +197,15 @@ class ArtistsControllerTest < ActionDispatch::IntegrationTest
           assert_equal("test", Artist.last.name)
         end
       end
+
+      should "warn about duplicate artists" do
+        create(:artist, name: "artist1", url_string: "https://www.pixiv.net/users/1234")
+        post_auth artists_path, @user, params: { artist: { name: "artist2", url_string: "https://www.pixiv.net/users/1234" }}
+
+        assert_response :redirect
+        assert_equal("artist2", Artist.last.name)
+        assert_equal("Duplicate of [[artist1]]", flash[:notice])
+      end
     end
 
     context "destroy action" do
@@ -213,6 +222,15 @@ class ArtistsControllerTest < ActionDispatch::IntegrationTest
         assert_redirected_to(artist_path(@artist.id))
         assert_equal(false, @artist.reload.is_deleted)
       end
+
+      should "warn about duplicate artists" do
+        @artist1 = create(:artist, name: "artist1", url_string: "https://www.pixiv.net/users/1234")
+        @artist2 = create(:artist, name: "artist2")
+        put_auth artist_path(@artist2), @user, params: { artist: { url_string: "https://www.pixiv.net/users/1234" }}
+
+        assert_redirected_to @artist2
+        assert_equal("Duplicate of [[artist1]]", flash[:notice])
+      end
     end
 
     context "revert action" do