We run loyalty program for customers.
When customers (both online and offline) purchase something, a field called " Loyalty point" is populated with the reward point that a customer obtained from their purchase.

Problem is: When a web customer registers online and makes a purchase via PayPal Express Checkout, the system creates a new customer record and populates the customer record with a new Loyalty point. If the customer has made a purchase before, they have an existing record with the Loyalty point field populated.

To deal with duplication, Mass Duplicate Update is used to merge customers with the same email addresses once a month.