FIX: Add check for existing provider_uids to generic import (#26914)

Co-authored-by: Gerhard Schlager <gerhard.schlager@discourse.org>

script/bulk_import/generic_bulk.rb

@@ -623,10 +623,13 @@ class BulkImport::Generic < BulkImport::Base
     SQL
 
     existing_user_ids = UserAssociatedAccount.pluck(:user_id).to_set
+    existing_provider_uids = UserAssociatedAccount.pluck(:provider_uid, :provider_name).to_set
 
     create_user_associated_accounts(accounts) do |row|
       user_id = user_id_from_imported_id(row["user_id"])
+
       next if user_id && existing_user_ids.include?(user_id)
+      next if existing_provider_uids.include?([row["provider_uid"], row["provider_name"]])
 
       {
         user_id: user_id,