Architecture & practice·Complexity and how it accumulates
you copied the block a second time and left it alone, and only pulled it out on the third.
Rule of three
Wait until you have three cases before abstracting over them. Two examples are not enough to see which parts genuinely vary, and an abstraction built on two is usually the wrong shape, which is worse than duplication because it must be unpicked before it can be fixed. It is the practical middle between religiously removing duplication and never noticing it.