I think the rule of 3 tends to lead to reasonable results, but asking "is this really the same piece of knowledge I'm encoding in N places" (as the original for

I think the rule of 3 tends to lead to reasonable results, but asking "is this really the same piece of knowledge I'm encoding in N places" (as the original formulation of DRY suggests) is going to be a little better. Sometimes it's two places but it's really clear they'll always change together, sometimes it's ten places but each is going to evolve independently (which, to be fair, might well be the determination made when "think[ing] about introducing an abstraction" in your formulation). To push back a bit on naive misapplication of DRY I've been saying we should call collapsing things that are just coincidentally similar (and likely to change independently) "Huffman coding".