Hi Cindy,
Here are some things to look for that can cause duplicates.
1) Two or more accounts that check the same mailbox.
2) Two or more inbox rules that act on the same message.
3) Antivirus/Antispam software that doesn't allow Outlook/Outlook Express to
complete the mail check cycle complete. (e.g. the antivirus/antispam causes
a timeout, so Outlook/OE doesn't logout of the mailbox correctly. Next time
in you get a repeat message.)
4) Some form of corruption in the mailbox. (e.g. messages with the message
id. you might be able to clear this if the ISP offers a web interface to
the mail account. otherwise the isp has to correct by removing the
offending message.)