The best method is to prevent Outlook from re-downloading the same email is to tell it to stop leaving mail on the server.  Here’s how to do it in Outlook/ Outlook Express:

• Browse to your email accounts settings (Tools -> Email Accounts – View or Change Existing)
• Click on your email account and hit “change” or “properties” at right
• Click on the “more settings” button
• Under the “advanced” tab you’ll want to uncheck “leave copies of messages on server”