For your collision box, add an additional 1 px collision box at the base of it. If this one overlaps the normal one (someone is standing on top of someone else), it pushes one or both rapidly off to a side until it is no longer overlapping.
If you don't want to use two separate boxes, you can also use is overlapping at offset condition to check below.
Thanks, that article gave me something to think about. I think you've led me to a functional solution for now using overlapping with offset. But I would like you to clarify what you mean by "add an additional 1 px collision box at the base of it". Would that be a single pixel or something at, say a pixel wider than the collision box? Would that be a solid box?