Bismart's customer master table may contain repeated email addresses from multiple imports. Write a SQL query to identify duplicate email values in the customers table.
| Column | Type | Description |
|---|---|---|
| customer_idPK | INT | Primary key for each customer record |
| full_name | VARCHAR(100) | Customer name |
| VARCHAR(255) | Customer email address, may contain duplicates or NULL | |
| created_at | DATE | Record creation date |