Well first, you need to understand that most POP3 configurations have a setting to leave a copy of your content on the server. This means that all of the saved data you have stored in your local .PST is still found on your POP server. This will create duplicates if when importing you did not select Do not import duplicates.

You mentioned you imported an .OST file. An OST file is created if you have an exchange account. Do you have 2 accounts configured within your Outlook client?

Best Regards,