The query you should will return every record in the Customers table. I think you're saying you only want distinct customers. In a well-designed database, that should be every record, but you can eliminate exact duplicates by adding the DISTINCT keyword:
SELECT DISTINCT * FROM Customers;
However, in the real world, the kind of duplication you get in a customer table isn't as simple as exact duplicates. It's having one record for Bob Space and another for Robert Space and a third for R. Space. This is a business problem, not a database problem and there's no simple query that's going to solve it for you.
If I haven't answered the right query, please explain what you were trying to ask.
Tamar