I doesn't usually happen like that for me. More likely, I stumble on a point where I have to reuse part of an old code, but with slightly different goals, or with some steps added/removed in the middle.

Copy-pasting and just dropping the useless parts is easier than making a new function, because you have to think about how to make the new function apply to both cases (the easiest way is "well, I'll just put a switch in the parameters and if statements", but it doesn't really lead to better code).