I see this all the time in being in the collision industry, this is one of my pet peeves when my kids get in my car or whoever, when they exit, to make sure the seat belt retracts all the way, and are not twisted, this will extend the life of your seatbelt greatly. There is no way to repair, or replace the tensioners in seatbelts, they are a non serviceable item, due to collision protection.
Hoss