Fix #4444: Pool navigation is broken when post appears twice in same pool.

Don't allow the same post to be added to the same pool twice.

This was only legitimately needed in a handful of cases. It was much
more common for posts to be mistakenly added to the same pool twice.

app/models/pool.rb

@@ -129,7 +129,7 @@ class Pool < ApplicationRecord
   end
 
   def normalize_post_ids
-    self.post_ids = post_ids.uniq if is_collection?
+    self.post_ids = post_ids.uniq
   end
 
   def revert_to!(version)

test/unit/pool_test.rb

@@ -233,9 +233,12 @@ class PoolTest < ActiveSupport::TestCase
       assert_equal("A_B", @pool.name)
     end
 
-    should "normalize its post ids" do
+    should "not allow duplicate posts" do
       @pool.update(category: "collection", post_ids: [1, 2, 2, 3, 1])
       assert_equal([1, 2, 3], @pool.post_ids)
+
+      @pool.update(category: "series", post_ids: [1, 2, 2, 3, 1])
+      assert_equal([1, 2, 3], @pool.post_ids)
     end
 
     context "when validating names" do