Please note that the latest update to outlook can cause some problems, and that this is not a server related problem.
This applies to pop3 accounts only. See referenced Microsoft article below.
One of the major complaints is that outlook is downloading any old emails multiple times.
Here are a few way’s that microsoft recommends to resolve this problem.
From Microsoft “
To stop receiving the same emails again you can follow the steps suggested below:
1. On the Tools menu, click E-mail Accounts.
2. Click View or change existing e-mail accounts, and then click next.
3. Select your ISP account, and then click Change.
4. Click More Settings.
5. Click the Advanced tab, and under Delivery, ‘uncheck’ the Leave a copy of messages on the server check box.
6. Click OK, and then click Finish.
You can also select one of the following options:
Remove from server after x days: E-mail messages are downloaded to your computer but remain on the e-mail server for the number of days that you specify. This is the most common setting for people who want to read their messages at work but also download them for permanent storage on their home computer. We recommend that you choose the smallest number of days that suits your needs. The longer you leave messages on your e-mail server, the greater the risk of exceeding your mailbox size quota.
Remove from server when deleted from ‘Deleted Items’: E-mail messages are downloaded to your computer but also remain on the e-mail server indefinitely until you delete the e-mail message in Outlook and empty the Deleted Items folder. Just deleting the message doesn’t remove the message from the e-mail server.
Related Microsoft thread
Outlook duplicate emails, duplicate mails outlook