newbie question: duplicate emails

Hello people, I am having a peculiar problem with my mailing list and hope you can help me out. I have a mySQL database with a table of email addresses for the mailing lists. I have many lists, and my (PHP) code runs various queries on this table to generate lists of email addresses to which the mail has to be sent. The email body is combined with this generated list, and then I use mail() to send this message out to each recipient. I also add on a few headers such as From:, Errors-To, Reply-To and Return-Path to the message. Now the problem is that every recipient of my mail is getting two copies of the message. I am positive that it's not my code which is causing the problem, the script runs once and terminates. I've also tried to send mail by opening a socket connection to port 25 on the mail machine and sending SMTP commands, but still the result is the same. And there is no SMTP error generated. Can anybody help me out here? or perhaps suggest a more efficient way of sending out mail to my users? I'm really in a spot here... - Nikhil