I see people doing this a lot:
My question is: Why?
When is it necessary to double up locking carabiners? Ever? Doubling up non-locking carabiners with opposite and opposing gates was, and still is a safe practice, but locking carabiners were an innovation invented to provide the same security, but eliminate the necessity of using two carabiners. Yet, there are people doubling up locking carabiners, as if they didn't have locks on them at all, and recommending it as best-practice. I support the notion that there is no such thing as overkill, but, if you're using a 25kN biner with a triple action gate, you are going to be plenty safe top-roping or using the biner to tie-in to your harness. So why are so many people strict on the practice of doubling up on lockers?