Messages being sent/received multiple times

[Abigail]

I quite often have people tell me that e-mails I've sent to them have
been received multiple times. The e-mails in question are not still in
my Outbox but the duplication seems to happen when I do a manual
Send/Receive (i.e. click Send/Receive vs automatic Send/Receive).

It may be something on their side (like their mail server not deleting
the message when they download it) but if it is something on my side
I'll like to figure out how to fix it!

Any ideas?

Thanks

Abigail