In case yer using Outlook Express for e-mail.
You probably have a corrupt POP3UIDL.DBX file.Close OE and delete that file.
It will be recreated when you re-open OE. One more download to get
pop3uidl.dbx up to date (OE uses this file to keep track of what has already
been downloaded), and you should be right.
Also, if you have more than one email account on the same POP3 server (same
ISP, different names) then keep on server will not work because of a known
OE bug.
Following is special instructions for Hotmail and Outlook Express:
When you download Hotmail, either do not access the Hotmail folder itself
whilst downloading, or turn off the option to include Hotmail in a
send/receive.
If you access the Hotmail folder (which initiates a download) at the same
time as OE is retrieving from the same Hotmail account, you can (and often
will) get two copies of the email.
Also see this link:
OLEXP: Hotmail Messages Duplicated [Q296447]
http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q296447