This one seems to have a good rating:
https://apps.shopify.com/doppelganger-app?locale=de&search_id=db3c81f7-48b5-4792-ba52-e577ea3f9007&surface_detail=duplicate+customer&surface_inter_position=1&surface_intra_position=4&surface_type=search
Let me know if that helped otherwise I will check for another solution 
Kind regards,