Firefox sending headers twice?

[nuwanda]

My php app is doing a couple of weird things: updating a database field twice (adding 1 twice, instead of once) and sending two emails for a single mail() command.

None of this happens when using IE7, only in FF2.x.

The DTD is XHTML-Transitional and it validates using the W3C validator.

I've used the FF Live Headers extension to view the headers and it definitely looks like page headers are being duplicated, which would account for the db and mail behavior.

I've googled and although I can't get a specific handle on the problem, it seems that similar problems occur with FF.

Is this a common problem, and what's the fix?

Thanks in advance.