There’s a simple fix to this problem. Most eCommerce platforms allow you to specify how many times a particular customer can use a coupon. By setting a maximum number of uses and separate coupons per social source, you can encourage customers to share through multiple different avenues.
Also, it’s a good idea in general to keep your coupons fresh.
Update: AddShoppers Pro now allows you to upload a list of one-time use coupons, so everyone who shares and receives a coupon will get a unique code that can only be used once. Most eCommerce platforms support bulk coupon code generation, so all you have to do is generate a list of coupons and then upload to AddShoppers in your Social Rewards campaign settings.
If you're not Pro already, you can upgrade here.