Heart Refill was one of the Power-Ups in Duolingo.
Heart Refill is a Power-Up in Duolingo that grants extra hearts during a lesson. One extra heart is given to users once it is purchased, and it will remain until it is used. It does not appear until the first heart is lost. You can get it for 4 lingots. It is only available on the Android Lingot Store.
Store description: Heart refill allows you to regain one heart lost during a lesson.
It gives an extra chance to complete the lesson without having to redo it, and will remain usable until used. However, it will not appear until the first heart is lost.
Users can also refill the third heart, if they only lose one during a lesson to get a full heart bonus. It takes 4 lingots to get the heart refill, and a full heart bonus will only reward users one lingot and one more experience point.