I'd like to add to point 1 a plea not to do the thing where people don't understand existing code so they copy it, partially modify the logic and then add branc

I'd like to add to point 1 a plea not to do the thing where people don't understand existing code so they copy it, partially modify the logic and then add branching to choose the correct version. It's fine in moderation to do this, but it increases your maintenance burden faster than it solves issues if you don't ever refactor.