I am with Russ, I have never heard of such a "rule". The axiom of "2 is 1, 1 is none" is really intended for critical items that have a tendency to fail. On a military mission, failure is not an option, you can't just turn around and say "I will try again tomorrow" so you carried redundant gear and accepted the weight.

In this vein, however; I would carry some redundant capabilities that are critical, but generally multi-use, such as shelter (ponchos), cordage, fire starting methods, tools (but generally different tools with different capabilities such as a sheath knife and a multi-tool), light (headlamp, larger flashlight, small pocket light). I may carry a little extra in the way of expendable supplies as well, but not double or triple.