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.