Comment by WorldMaker

> You want to extract common code if it's the same now, and will always be the same in the future.

I suppose I take that as a presumption before the Rule of 3 applies. I generally assume/take for granted that all "exact duplicates" that "will always be the same in future" are going to be a single shared function anyway. The duplication I'm concerned about when I think the Rule of 3 comes into play is the duplicated but diverging. ("I need to do this thing like X does it, but…")

If it's a simple divergence, you can add a flag sometimes, but the Rule of 3 suggests that sometimes duplicating it and diverging it that second time "is just fine" (avoiding potentially "flag soup") until you have a better handle on the pattern for why you are diverging it, what abstraction you might be missing in this code.