Not really much of an issue with SXDL any more, and even SD1.5 got quite good at the end. Admittedly haven't stress-tested either, though (things like clasping hands etc). It's also not a hand-specific thing, it just happens more commonly with fingers because they're small features:

The thing that happens is that diffusion-type interference first nails down gross structure (which limb is where) and then fills in details. Sometimes steps somewhere in the middle decide that a limb should be somewhere else, though, and suddenly you have two, and if steps immediately after don't think "that old limb doesn't look like it should be there" and erase it, later steps will happily refine both to photorealism because they don't even look at the overall composition. That is, it's not an issue with anatomical knowledge, or not having seen enough hands, but the model changing its mind but not backtracking. It's actually astonishing how good it can get at not making that mistake without being able to tell that it has two competing goals in mind.