mysql_insert_id problems

I have a system which shoots off an email with a reference number every time a record is entered into MySQL. The reference number is formed from the value of an auto_increment field using mysql_insert_id(). However yesterday a few cracks started appearing - it's sent out emails containing the same reference number twice, though the database looks sound (no problems found with isamchk) and it's only inserted one record (the other one is lost, not just inserted with the next available ID). Are there any known problems with mysql_insert_id() that might cause this to happen? Thanks Chris Newman Lightwood Consultancy Ltd chris@lightwood.net