I don’t have anything really insightful here I’m afraid.
First, if the boxes are not truly aligned (position.x and size.width) I would expect this behaviour.
Second, if they are aligned, then I believe it simply might be the fact that Box2D does not collapse the collision points. This is a common problem in all physics engines, and they do various things to solve this (e.g. merging geometry).
I don’t know Box2D enough to suggest a good solution for this. Ping @Ragnar_Svensson, do you have any good ideas?