diff --git a/app/models/collection_item.rb b/app/models/collection_item.rb
index 8877f3f8da..c2ba2e9b68 100644
--- a/app/models/collection_item.rb
+++ b/app/models/collection_item.rb
@@ -27,6 +27,7 @@ class CollectionItem < ApplicationRecord
 
   delegate :local?, :remote?, to: :collection
 
+  validates :account_id, uniqueness: { scope: :collection_id }
   validates :position, numericality: { only_integer: true, greater_than: 0 }
   validates :activity_uri, presence: true, if: :local_item_with_remote_account?
   validates :approval_uri, presence: true, unless: -> { local? || account&.local? || !accepted? }
diff --git a/db/migrate/20260410083500_add_index_to_collection_items_account_id_collection_id.rb b/db/migrate/20260410083500_add_index_to_collection_items_account_id_collection_id.rb
new file mode 100644
index 0000000000..35acd93ec0
--- /dev/null
+++ b/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
diff --git a/db/schema.rb b/db/schema.rb
index 70810b10d9..6dddbc0efa 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_03_26_112324) do
+ActiveRecord::Schema[8.1].define(version: 2026_04_10_083500) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
@@ -370,7 +370,7 @@ ActiveRecord::Schema[8.1].define(version: 2026_03_26_112324) do
     t.integer "state", default: 0, null: false
     t.datetime "updated_at", null: false
     t.string "uri"
-    t.index ["account_id"], name: "index_collection_items_on_account_id"
+    t.index ["account_id", "collection_id"], name: "index_collection_items_on_account_id_and_collection_id", unique: true
     t.index ["approval_uri"], name: "index_collection_items_on_approval_uri", unique: true, where: "(approval_uri IS NOT NULL)"
     t.index ["collection_id"], name: "index_collection_items_on_collection_id"
     t.index ["uri"], name: "index_collection_items_on_uri", unique: true, where: "(uri IS NOT NULL)"