<?php
global $wpdb;

$entry_table_name = $wpdb->prefix . 'bw_winners_entries';
$query = "ALTER TABLE $entry_table_name DROP INDEX import_id;";

$wpdb->query( $query );
if ( $wpdb->last_error ) {
	if ( $wpdb->last_error = "Can't DROP 'import_id'; check that column/key exists" ) {
		$wpdb->last_error = false;
	} else {
		return $wpdb->last_error;
	}
}

return false;