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

Ilya's answer is correct, I just wanted to add a little more than would fit in a comment:

If resubmission is dangerous (going back and submitting again, reloading the result page [if you haven't taken Ilya's advice], etc.) I use a "nonce" to make sure the form can only go through once.

On the form page:

// ... snip ...

In the processing page:

After the form has been submitted once, it cannot be submitted again, unless the user intentionally fills it out a second time.