I found a solution; use pre_get_posts:
add_action( 'pre_get_posts', 'modify_attachments' );
function modify_attachments( $query ) {
if ( is_admin() && strpos( $_SERVER[ 'REQUEST_URI' ], 'admin-ajax.php' ) !== false ) {
add_filter( 'posts_groupby', 'group_attachments' );
}
return $query;
}
function group_attachments( $groupby ) {
$groupby .= " guid";
return $groupby;
}