Query failed! Duplicate entry '0' for key 'PRIMARY'

Backtrace:
Array
(
    [0] => Array
        (
            [file] => /home/chessman/public_html/webchess/includes/classes/Database.class.inc
            [line] => 80
            [function] => write_query
            [class] => Database
            [object] => Database Object
                (
                    [queries:protected] => Array
                        (
                        )

                    [read_count:protected] => 3
                    [write_count:protected] => 2
                )

            [type] => ->
            [args] => Array
                (
                    [0] => INSERT INTO vacation (`playerID` ,`vDays` ,`vStart` ,`vEnd`) VALUES ('-1',  '30',  '0',  '0')
                )

        )

    [1] => Array
        (
            [file] => /home/chessman/public_html/webchess/vacation.php
            [line] => 41
            [function] => query
            [class] => Database
            [object] => Database Object
                (
                    [queries:protected] => Array
                        (
                        )

                    [read_count:protected] => 3
                    [write_count:protected] => 2
                )

            [type] => ->
            [args] => Array
                (
                    [0] => INSERT INTO vacation (`playerID` ,`vDays` ,`vStart` ,`vEnd`) VALUES ('-1',  '30',  '0',  '0')
                )

        )

)