If you have multiple email hosts, e.g. Mac, gmail, hotmail, work, and you forward any of them you can set up loops that duplicate emails. Try canceling any forwarding temporarily and if it fixes problem only forward one at a time until you find the problem. I was getting four copies of some emails and this is how I solved the problem. Without any firm proof I felt that forwarding from .Mac (Mobile Me) was a major problem as was forwarding from my University mail system.
Recently I have started receiving two copies (both dated and received at the same time) of many Mac related emails.
I have contacted many of the senders and also my ISP but in each case only one copy was sent in each case.
I have trashed my preference file and checked out everything else I can think of on my eMac, but I'm still getting two copies.
Has anyone any ideas how I can correct this please?
I am using an Emac with a PPC G4 proceser, running Mac OSX 10.4.11 using Mail 2.1.3. I have 1GB of memory with 2@512Mb mem. chips.