Data sent twice or more in forms?

Ever having with a form beeing sent twice doubling your data with the insert command in sql unwanted? LIke when a user refresh the webpage or so..
One solution is to have a form token.... blabla
Also there's a simplier way that works for as a quickfix, after the page that has inserted the data has loaded send the user to a new page like this

header(sprintf("Location: thanks2.php"));
?>

1st: form-file.php
2nd: the-sql-insert-file.php
3d: thanks.php