What happens if the same coupon is applied to a customer twice

Whether Lago allows a coupon to be applied twice to the same customer depends on the coupon's reusable setting. Non-reusable coupons block a second application; reusable coupons allow it, and the discount stacks.

How it works

Non-reusable coupons (reusable: false) can't be applied more than once to the same customer. A second attempt through the API or UI returns a validation error (coupon_is_not_reusable) and is blocked, so only one instance is ever active. Reusable coupons (reusable: true, the default) can be applied multiple times, with each application treated as a separate instance that's applied at invoice finalization. Two applications of a $300/month coupon result in $600 off per invoice, and there's no automatic deduplication.

Checking and preventing stacking

To see what's active, check the customer's Applied Coupons section in the UI, or call GET /api/v1/applied_coupons filtered by customer. Remove an unintended duplicate with DELETE /api/v1/applied_coupons/{id}. To prevent stacking going forward, set reusable: false when creating the coupon, or uncheck "The coupon can be applied several times to the same customer" in the UI. Lago then enforces the uniqueness constraint automatically.

This article reflects guidance drawn from customer case resolutions. It is not officially supported documentation and may not apply to all situations.