plugin create multiple coupon for the same email address

Hello,

I have just realized that the plugin is sending coupon twice to subscribers who clic the “subscribe to our newslwtter” box at checkout. Allthough they already subscribe to the mailing list, they receive a new coupon.

How can I prevent this to happen ? it seems that whatever if the customer has already subscribe or not, the customer gets a coupon everytime he checks out.

Thank you