Fix collections allowing multiple occurrences of the same user (#38636)

db/migrate/20260410083500_add_index_to_collection_items_account_id_collection_id.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+class AddIndexToCollectionItemsAccountIdCollectionId < ActiveRecord::Migration[8.1]
+  disable_ddl_transaction!
+
+  def up
+    add_index_to_table
+    remove_index :collection_items, [:account_id]
+  end
+
+  def down
+    add_index :collection_items, [:account_id]
+    remove_index_from_table
+  end
+
+  private
+
+  def add_index_to_table
+    add_index :collection_items, [:account_id, :collection_id], unique: true, algorithm: :concurrently
+  rescue ActiveRecord::RecordNotUnique
+    remove_index_from_table
+    deduplicate_records
+    retry
+  end
+
+  def remove_index_from_table
+    remove_index :collection_items, [:account_id, :collection_id]
+  end
+
+  def deduplicate_records
+    safety_assured do
+      execute <<~SQL.squish
+        DELETE FROM collection_items
+          WHERE id NOT IN (
+          SELECT DISTINCT ON(account_id, collection_id) id FROM collection_items
+          ORDER BY account_id, collection_id, id ASC
+        )
+      SQL
+    end
+  end
+end