If you are using DonationMatch system-generated gift certificates, you will be able to track usage of each one by its assigned certificate number.
When you are presented with a certificate, you will want to take the certificate # (printed on the left, bottom third of the certificate) and input it through the Verify/Redeem Certificates button on your dashboard, within your TO DO box as shown:
Entering the number will tell you whether it has been used before or not AND give you the option to mark it as used.