You are absolutely right, and I don't know either. I just add zebra nodes. Sometimes I shift some nodes a bit, but that's about it! I can imagine a duplication happening, or an accidental shift of a way, but adding hundreds of nodes to one way, I can't see how I could have done that. Anyway, I'll ask around how to analyse/repair this, given the fact that it's the outer of a multipolygon, and it's attached to many buildings including entrances.