The best part is, it's all handled for you. Whether you have duplicates in your uploaded file, or you've imported the same contacts to the same list, there will never be duplicates, or copies, of contacts in your subscribers area.
If you have repeated contacts in any file that you import to your subscribers, you don't have to sweat it. Email Marketing will notice those repeats in there, and simply won't import any contact more than once. You'll end up with less contacts in your specific subscriber list than are listed in your original file, since those repeats will be ignored.
If you are importing contacts to an existing list that already contains all or some of the same contacts, no need to worry! Email Marketing will recognize the overlaps and never make any duplicates of any contacts within a single list. You could import the exact same file to the same subscriber list 100 times, and those subscribers would still be in that list in your account, only once.
All the other information associated with that contact (i.e., name, company, phone, etc.) will be updated to reflect the details in your latest upload. So, if you ever need to update your contacts' info en masse, just change it all in a spreadsheet file with the email addresses included, and upload it to your Subscribers. A subscribers export is the easiest way to get a perfectly formatted spreadsheet to work with.
Keep in mind that you CAN have the same contact show up in multiple lists. This does NOT mean that the contact is duplicated, it merely means that it is categorized in more than one way. It's helpful to think about your All list as a master, total list. And to think of all your other lists as simple filters, or lenses, for different ways of looking at that single, master list.