While migrating old data to a new database schema I was unable to activate a PRIMARY KEY on the legacy_customer_id field because there were duplicates. It turned out that the old application did not clean the data really well, and it allowed duplicates to be created where one of the customer_ids had a space character making it unique.
I used the following query to test for others:
SELECT customer_id, COUNT(customer_id) FROM customers GROUP BY customer_id HAVING (COUNT(customer_id) > 1);
This allowed me to find all customer_ids that had duplicates and clean things up.