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Outlook duplicate records can occur for many reasons, and in most cases, a simple fix will prevent them from happening in the future. Below you will find some of the most common causes of duplicates, and some solutions to prevent them in Outlook.

The first, and probably the most common, is synchronization of your Outlook with a PDA. Duplicates can occur here if you have the same Outlook record stored under different terms in Outlook and your PDA. For example, if you have the same contact (we'll call him Bill Henderson) in both your PDA and Outlook, and Bill's job title in your PDA is "Sales", but in Outlook he is listed as "Sales Rep", when you synchronize your PDA to Outlook it will create an Outlook duplicate record: one for Bill Henderson "Sales Rep", and another for Bill Henderson "Sales".

To prevent this, one of the easiest solutions is to get your Outlook contacts in order. You can do this by creating a new secondary Outlook account (not configured for email) and sync all of your contacts from your PDA into this account as new. You can then clean them up to match your original Contacts folder, and sync back into your PDA. Once that’s complete, you can delete the secondary Outlook account and your next sync to Outlook should go flawlessly.

Another reason for Outlook duplicate records could be that Outlook downloaded an email twice from one or more POP3 servers. This could happen for multiple reasons; a background synchronization happened at the exact time you were emptying the Deleted Items folder, messages you intentionally leave on the mail server are downloaded again on the next Send/Receive, you have two or more alias accounts pointing at the same POP3 server, or other inconsistent occurrences.

If you believe your Outlook duplicates are caused by an underlying Outlook functioning error, check with Microsoft Support at http://support.microsoft.com/; in many cases, you just need to update your software because of an Outlook bug.

Reinstallation and re-configuration of Microsoft Outlook could also Outlook duplicate records. If you install a fresh installation of Outlook with your email account, select your old .pst file as the delivery location, but originally chose to leave a copy on the server; those old emails will not only still be in your .pst file, the server will also download them again and create Outlook duplicates you will need to remove. This is because the server didn't recognize that the emails had already been received by another email account.

To prevent this problem, make sure you start Outlook with a clean .pst file after re-installation. Once the "new" email messages have been received, open your old .pst file and move only the new messages (not the old message "duplicates") into the old .pst file. Then delete the new .pst file and set your default delivery to the original .pst file.

Your firewall and/or anti-virus software can also create Outlook duplicates. In a perfect Send/Receive cycle, Outlook sends a request to the mail server for mail messages; new emails not in Outlook are then downloaded and the connection is closed. However, if your firewall or anti-virus software interjects itself and causes something to fail at any point in this process (flagging a large message, etc.) the connection will not close properly. This means that any emails that were downloaded before the point of failure will be downloaded again on the next Send/Receive, thereby creating duplicate emails in your Outlook inbox.

If you think your firewall/virus scanner is causing Outlook duplicates, change your security settings to not interfere with Outlook. Outlook's "in-house" security measures are tight enough as it is, and it does the work for you, so there’s no need to worry.

Check your rules in Microsoft Outlook – they could be set to send the same message to two or more places. If you have multiple rules set up to deal with incoming email messages, there's a possibility that Outlook could create duplicates in order to satisfy two different rules. For example, if you have a rule to send all emails from "Jack Morris" to a specific folder, and all emails with the subject line "Outlook" to go to the same folder, and you get an email from "Jack Morris" with "Outlook" in the subject line, technically both rules apply. Outlook will then move the message twice, creating a duplicate email that you will want to remove.

To prevent this problem, be sure to add the action "stop processing more rules" to the rule. This way, if a rule has already been applied to a message, no further action will be taken and duplicates will not be created to satisfy all rules.

REFERENCE

http://www.anti-dupe.com/products/prevent_outlook_duplicate.aspx