Display a field list from all records to aid in data entry to prevent duplicates

Display a field list from all records to aid in data entry to prevent duplicates

I have an "Add new customer" form with a Single Line text input field to get the customer name but to prevent users from accidentally adding the same customer which existed already I want to show them a list of existing customers from the same form.

What's a good way of doing that. I tried a formula but that only refers to the current customer. I want a list of all customers already in the db to be displayed maybe next to the entry field. 

Thoughts?