You might check (Tool - Accounts in Windows Mail) to see if you have a duplicate email account setup in your email program. If you do, then just delete the duplicate to stop the duplicate emails.
Nope and to prove I am getting senile I forgot to state it doesn't happen all the time. Kinda like the person with a intermittant miss in his engine. Hard to find when it isn't happening. Good idea I will compare the headers the next time it happens. It only happens to emails about posts so I know it is not a problem with my email settings. Maybe I am just shell shocked
I did spot a setting in the forums mail settings that had a note next to it saying "If you have a large site, you may wish to enable. This prevents a rare situation where the same email is sent multiple times"
This could have been the problem and is now enabled.
Please let us know if you have any more duplicate emails.