Unfortunately, I offer to help. I have the same problem but using Outlook. It doesn't happen all the time, but I'd still like to find out why. I have Trend Micro anti-virus and firewall and sometimes I use my own smtp server. Doesn't seem to make any difference.
I have a Compaq Presario 6400NX running XP Home. It has 40 Gig Hard drive and 352 MB of DDR RAM. I use Outlook Express and have never had any problems until a month ago. I ran McAfee virus scan weekly with no viruses reported and changed to ZoneAlarm anti virus on mid-August, again running it weekly with no viruses reported. I have had ZoneAlarm firewall for 3 years. I checked my ISP, Cox.net (cable)and they said they had no problems and the sample emails I sent them came through singly, not in duplicate as most of my email corresponders report back to me.
I have checked all my connections and setups on email and they match the requirements of my ISP. I am not an expert in computer science and this problem is perplexing the hell out of me!