PushSend identifies contacts by their email addresses. So you will never have two contacts with the same email address in your lists. When you upload or manually add a contact to the List Manger, the system will check to see if that email address already exists. If that contact is already in the List Manager, it will be not be added a second time. But it will be updated if there is additional information.
The only way a contact could be duplicated in the List Manager is if it is entered with different email addresses. In this case, you can search by name in the List Manager to find duplicates and delete one if you choose.