Imported Mailing List - Call-in Lead (How to Avoid Duplicate Entries)

I import my leads using a spreadheet. Then, I send out a direct mai lmarketing campaign to those people. If they call in, I have an answering service that takes the call and enters the information into the system as a lead. how do I avoid having a duplicate entry for a lead if the person that calls in has already been imported into the CRM via the mailing list?

Thank you