I know it's three years after this thread got started, but it still comes up in the first page or two of Google searches for "Entourage duplicate email", so here's what little I've been able to figure out. There are multiple, not necessarily related causes for this problem:
- This problem happens with POP servers as well as Exchange servers, but sometimes for different reasons, requiring different fixes.
- Budge's experience above, implies that at least part of the time, Entourage might sometimes not properly maintain its internal database of which emails its already received (corroboration, at least for earlier versions of Microsoft email apps, is at
http://www.insideoutlookexpress.com/problems/bugs.htm#pop3uidl) (though unfortunately there isnt an equivalent to the "Pop3uidl.dbx" file for Mac/Entourage users).
- When I first looked into this problem a couple years ago, I first thought Entourage might sometimes not notify the email server that it's received email, but I learned thats not how mail servers operatethey dont receive any such notification from email clients that access them, except to delete mail that the server has just sent (depending on your email app's setting for when to delete email), but Ive located at least one report from an Entourage user who found that even turning on the option to delete mail from the server as soon as Entourage downloads it, didnt help (implying for that user, email really wasnt being deleted from the server), so this again implies the problem is often (usually?) Entourage (and sometimes other email apps too) not properly maintaining its own database as to what emails its received.
- Some people have at least temporarily fixed the problem by deleting their Entourage email account, and re-creating it. But since a simple deletion of your Entourage account/identity will lose your email, I suspect a better solution would be to export your identity's data, create a new identity, then import that data, since I don't think the bug is in any of Entourage's prefs files, but instead in the identity's "Database" file.
- I tried to determine whether the problem is in a specific prefs file associated with Entourage that can be trashed and re-created, without disturbing Entourage's Database file, but I have my doubts whether its in any of the simpler prefs files (~/Library/Preferences/ com.microsoft.Entourage.plist, or ~/Library/Preferences/Microsoft/ Entourage Preferences, etc.), since I dont see them changing their modification date when you send or receive email. That's why I think Entourage may track downloaded email in its catch-all Database file at ~/Documents/Microsoft User Data/Office 2004 Identities/Main Identity (or whatever your account is named), which would be a pain since I cant see a way to repair it of this problem (using Entourage's rebuild option doesnt fix the problem, though it may sometimes remove duplicate emails in your inbox, until the problem happens again; and, manually editing the Database file with a file editor will probably screw it up), except maybe by exporting its data, as I describe above, and importing that data into a new identity. There may be some message tracking done by the cache of message pointers Entourage maintains at ~/Library/Caches/Metadata/Microsoft/Entourage/2004/Main Identity/Messages, but deleting any of the pointer files in the folders inside this folder, doesnt cause Entourage to re-download any of the associated messages when you tell it to receive messages. Its not in the file named index in the Messages folder at that path.
The alternative is to use a different email app. I use Apple's Mail, but I work with clients who use Entourage, and are seeing this problem still, with OS 10.4.11, and the latest Office 2004, version 11.4.1, so it seems Microsoft doesn't think we're good enough to have this fixed.