Yes, when you upload a new list the system scans your list and removes duplicates from your file.
In addition, when you upload new members onto an existing list, the system will scan for duplicates, bounces, and unsubscribes that are already stored in your account as well.
Note: lists are independent of each other and do not share information, so the system does NOT search for duplicates, bounces or unsubscribes across multiple lists.