Check duplicate customer Field

Hello,

How can i create a customer field with same check duplicate feature as the Name Account?

I have created the field i only don’t find a way to check the duplicate values.
When i insert the value in the field and save will be nice check if have already have the same value in the database (the Name Account do this)

Anynone have this issue in the past? can help me?

Best regards,
Hoko