Stop data inserting into a database twice

Question

I was wondering what methods/preventions other programmers use to stop data being entered twice into a MySQL database when a user refreshes on the same page as a form? Obvio

Answer 1

I call this a golden rule of web programming:

Never ever respond with a body to a POST-request. Always do the work, and then respond with a Location: header to redirect to the updated page so that browser requests it with GET.

This way, refreshing will not do you any harm.

Also, regarding a discussion here in comments. To protect from double posting from, say, accidental doubleclicking the Submit button, store an md5() of your form in a text file, and compare the new form’s md5 to the stored one. If they are equal, you are having a double post.