I keep getting duplicate messages in my Inbox!?!

1. Tools | Account Settings.  How many accounts do you have set up there?  What kind of account is e-mail? (POP3?  Exchange? Something else)  Make sure you don't have it defined twice - for example an Exchange e-mail account you're also checking via POP3 or IMAP.

2. Rules Wizard.  Do you have any rules defined that might be making copies of messages?

3. Antivirus.  If neither of the above are the answer, make sure your antivirus software isn't set to scan incoming e-mail. Sometimes that process can create duplicates and/or blank messages.