I think I remember a Carmack tweet where he mentioned in most cases he only considers it once he reaches three duplicates
I once heard of a counter-principle called WET: Write Everything Twice.
Another law for the list!
Why 3? What is this baseball?
Take the 5 Rings approach.
The purpose of the blade is to cut down your opponent.
The purpose of software is to provide value to the customer.
It's the only thing that matters.
You can also philosophize why people with blades needed to cut down their opponents along with why we have to provide value to the customer but thats beyond the scope of this comment
The "Rule of 3" is a pretty well known rule of thumb; I suspect Carmack would admit it predates him by a fair bit.