mysql problems

Hi all - I could use a lead on a problem. I just don't know where to start. I have a PHP script that populates a database table. No big deal. It creates mailing labels. However, a weird things keeps happening - every once in a while, a query is run twice. It is the same query, same information, even the same time (there is a now() in the query - and it is identical). So the question is a simple one - is this a PHP problem or a MySQL problem? Or somewhere in the MySQL extension? And how would I know? There is one clue to this otherwise vague problem. I believe that this predominantly happens when the database is under an above average load. I would appreciate any help that I might be able to get. Thank you.