I am receiving duplicate emails in my windows mail inbox. How can I stop this?

Do these three things checking as you go,

Remove the account, reboot and then add the account back.

Try compacting and repairing the database.

WMUtil

http://www.oehelp.com/WMUtil/Default.aspx

Turn off e-mail scanning in your anti-virus program. It is a redundant layer of protection that eats up CPUs, slows down sending and receiving, and causes a multitude of problems such as time-outs, account setting changes and has even been responsible for loss of messages. Your up-to-date A/V program will continue to protect you sufficiently. For more, see:

http://www.oehelp.com/OETips.aspx#3 

Why you don't need your anti-virus to scan your email

http://thundercloud.net/infoave/tutorials/email-scanning/index.htm

The most efficient way to do this is to completely remove the anti-virus program and then do a custom installation. When the opportunity to opt out of e-mail scanning arises, opt out.

Thank you, Bruce!  We fixed it.

Working through the remove/reboot and WMUtil steps above, I stumbled on the source of the problem.  The duplicate-message e-mail address had found its way into one of the other accounts.  So, checking the other account also collected messages from the problem account. 

WMail popped up a sign-in box during in the wee hours of Wednesday morning, as it is wont to do every once in awhile.  Usually, we just cancel and life goes on.  But, late at night, the box triggered the respose that used to be required when we had DSL -- signing in with the e-mail account that suddenly got sick.

The result of that sign-in was to put the problem-account e-mail into a second account.  Both accounts were checking the same 'post office box' and then sorting the incoming to the same inbox. 

Correcting the second account fixed the problem.

Thanks to Bruce for good advice and patient coaching.

YW. Glad you got it resolved.