This error message means is that the email address is already on a mailing list within your account. Once an email contact is in your account, you can just select the lists you want to add it to, no need to upload again!
To do this, click on the email contact that you want to add to another list.This will take you to the List Memberships tab where you'll see a checkmark next to all the lists that contact is on. From here you can add the contact to a list(s) by clicking the box, or, click to remove the checkmark to remove the contact from a particular list. And again, be sure to click Save if you want to save your changes.