If you are receiving duplicates of only some of your mail, the cause may be technical difficulty along the delivery path. Either a network connection is having problems, or one of the computers along the route is very slow or having other system problems.
Most mail delivery protocols and programs are conservative. If they are unsure whether a mail message was successfully passed along to the next computer on the network, the sender will try to send it again. If the message was passed along successfully the first time, the second attempt will produce a second copy of the same message, and you will likely receive both copies.
On large networks such as the internet, a mail message must usually pass through several different computers before it reaches its final destination. This problem can potentially occur at any computer along the way. A synchronization problem in the SMTP mail delivery protocol, which is used by most mail programs on the internet, can make this error more likely; see the internet RFC document Duplicate Messages and SMTP.
Unfortunately, this problem has no real solution, except to wait until the problem on the network is resolved. If this problem is chronic and doesn't seem to be associated with a more general network or computer problem, the mail delivery software on the computer in question may be misconfigured.