Has anyone created a flow automation that can detect duplicate customers once a customer is created and then merge them or notify me to merge them? Thanks!
I would recommend using an app for that, it would stay in the backend and not slow down the shop itself.
Also, apps like that will check many factors to find out if there are potential duplicates.
What app do you suggest for this?
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,
According to reviews the app has a very bad rating and seems it does not do what the purpose is