PHP Mail Duplicate Emails

I am using the integral mail function to send email from one of my sites. When I debug while sending email, I see my function run only once. The function also echo’s out the single distinct multipart identifier used in the one function call.

When the email arrives in my inbox, I have two copies. Same content. Except… The second email has a different multipart identifier. My function never generated the second identifier.

I am stumped. I have been searching and have made sure I am not doing something stupid like including a "To: " line in the header, I am not using CC: or BCC:.

Some google results indicated that a hosting companies spam filters can do things like this… The email is being delivered to my domain email, which is served by Google Mail.

Thanks for any help!