This really isn't a perl question.

When you refresh the browser it will normally resubmit any information to the server and thus cause your problem.

There are a number of ways around this. Ususally I just put a big warning saying "DO NOT RELOAD THIS PAGE" or "ONLY CLICK SUBMIT ONCE OTHERWISE YOU WILL BE CHARGED TWICE."

One other method you can use is to generate a time stamp on the form. When it is submitted if the difference between now and the timestamp is too great, give them a time out error. If data for that timestamp has already been entered then tell them they've already click submit once.