Heart Refill was one of the Power-Ups in Duolingo.
Heart Refill was a Power-Up in Duolingo that granted extra hearts during a lesson. One extra heart was given to users once it was purchased, and it would remain until it was used. It would not appear until the first heart is lost. It was discontinued after the heart system was discontinued outside of testing out of a skill.
Store description: Heart refill allows you to regain one heart lost during a lesson.
It gave an extra chance to complete the lesson without having to redo it, and would remain usable until used. However, it would not appear until the first heart was lost.
Users could also refill the third heart, if they only lose one during a lesson to get a full heart bonus. It took 4 lingots to get the heart refill, and a full heart bonus would only reward users one lingot and one more experience point.