1. Make sure that there is no duplicates email account setup on the same MS Outlook. 

2. Make sure that there is only one MS Outlook is running. Check on the task manager to make sure that there is no duplicate outlook.exe instance running on the same machine. Duplicate instances of outlook.exe running can produce duplicate messages arriving at the same time. You will need to then determine why two instances of Outlook were running in the first place.

3. Make sure that you do have Service Pack 2 installed if you are using MS Outlook in Windows XP.

Between to fix the problem, please try:

1. Close the Ms Outlook to disconnect the POP3 access (This is to prevent your email account being locked in the server)
2. Login to your webmail account
3. Check the inbox and Remove the unwanted emails, spam mails or large size emails
4. Login out the webmail
5. Reopen the Ms Outlook and try again to receive the emails via POP3 connection