Duplicate Lookup calls have the exact same data as a previous Lookup (essentially, a cached call). Each Lookup is 'fingerprinted', so a duplicate call is an exact replica of an API call that's already been sent to TaxCloud. The problem with duplicate calls is that they slow down the system unnecessarily.
Duplicate/ redundant calls can happen for various reasons. For example, please make sure your shopping cart is not re-sending a Lookup request each time a customer moves from one page to another, after having placed an item in his/her shopping cart.
It shouldn't be hard for your developers to figure out why the shopping cart is re-sending the same Lookup.