How would you repair this?

Double size of plate,and drill/plug weld two rows of spot welds around plate Will never fail as long as you own it.

imagine a lap joint with one weld along edge. Work it a little and it will break. Having 2 rows of welds and metal is stabilized in such a way that it can no longer flex at weld points.

next time you get near an airplane, look at the rivets. There is 2 rows of rivets holding it together.