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

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? }