problem with mysql / auto increment fields.. ?

On my website there are a couple places where people can sign up .. The querys after the sign up process look like $blahblah = query(insert firstname lastname) values (blah blah blah) $userid = mysql_insert_id($blahblah); $insertintoothertable = query(userid, blah blah blah) etc. it then uses this userid var to insert them into a variety of other tables for other things on the site, such as a phpBB account, etc. if multiple people are signing up for accounts at different places, is there the possibility that a duplicate userid could be assigned, or anything like that? Thanks, Chad