Group filter is not working for Firstname

Content

Hi,

We need to develop the report that who are all having the duplicate[same] firstname. So we try to develop the report by using the group filter where we mentioned count(contacts.first_name) > 1. But it is not working correctly. When we try to put the group filter for duplication email ,it is working fine. so kindly provide the solutions for finding the duplicate first name.

 

Version

Service Colud Feb 2016