> A test I like to use for whether two things are actually or just incidentally related is to think about “if I repeat this, and then change one but not the other, what breaks?”

I really like this question as a way of figuring out whether things happen to look the same or actually should be the same for correctness, plus it feels like it should be an easy question to answer concretely without leading you down the path of 'Well we might need this as a common component in the future'.

I also think you can frame it as a same value or same identity type question.