The baseline for identifying your customers is set when you initially create your Check This Out organization account and add your list of customers or when you add customers manually or via import during the course of business. From then on the following definitions apply,

New Customer

A Customer who redeems a coupon who's contact information does not match one of your existing Check This Out contact records.

Repeat Customer

A customer who redeems a coupon who's contact information matches one of your existing Check This Out contact records.