There are two ways to see what contacts are on what lists.
The first way sounds like you are already using this method and that is to do a GET on the individual email address and read the list array to see which lists are present.
The second option would be to check the list itself and see what contacts are present.
Depending on your database setup they could both have pros and cons. If you find that you are using the most efficient method for your setup and you are running in to your daily limit please email firstname.lastname@example.org with some details about your situation and we can go over some options for you.
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