Cognitive load is what matters

This was my main takeaway from A Philosophy Of Software Design by John Ousterhout. It is the best book on this subject and I recommend it to every software developer.

Basically, you should aim to minimise complexity in software design, but importantly, complexity is defined as "how difficult is it to make changes to it". "How difficult" is largely determined by the amount of cognitive load necessary to understand it.

Which is why I consider DRY (Don't Repeat Yourself) to be an anti-rule until an application is fairly well understood and multiple versions exist. DO repeat yourself, and do not create some smart version of what you think the problem is before you're attempting the 3rd version. Version 1 is how you figure out the problem space, version 2 is how you figure out your solution as a maintainable dynamic thing within a changing tech landscape, and version 3 is when DRY is look at for the first time for that application.

Closely related to the Rule of Three - ok to duplicate once, but if it is needed a third time, consider refactoring: https://en.wikipedia.org/wiki/Rule_of_three_(computer_progra...

I think it's a pretty good compromise. I have tried in the past not to duplicate code at all, and it often ends up more pain than gain. Allow copy/paste if code is needed in two different places, but refactor if needed in three or more, is a pretty good rule of thumb.