While duplicate emails are not allowed within one list, you are allowed to have one email in more than one of your lists. We recommend having one master list for all your contacts and use segments to group them. This way you won't have to pay extra for duplicate emails stored in your account.
Combine Your Lists in One
You can combine your lists using the export/import method.
Start by choosing one list where you will import all the other lists. You can just choose the list with most contacts in it.
Export the data from all the other lists. A step by step guide on how to export the list is available here.
Convert files from .xlsx to .xls You can use an online conversion tool like this one to do that.
Import the files in your main list.
Don't forget to tick Update existing subscriber details box. A step by step guide on how to import lists is available here.
Afterwards, you can simply delete the unwanted lists.