It seem that this module - https://apps.odoo.com/apps/modules/9.0/crm_duplicates/ - would be of use

Hi, I think it will better open pgadmin and look your duplicate customer

SELECT name FROM res_partner GROUP By name HAVING ( COUNT(*) > 1 )

then you will find all the customer which is more then one in entry in customer table, and after this go in relevant python file of customer and put _sql_constraint for name or name or code combination so next time when you upload or create the same entry it will give a pop-up and will be not allowed to enter duplicate customer name.

Understood. Thanks!

You have to do it manually or if you have access to postgresql you can make a SQL sentence to delete all the table