Redundancy makes sense in some situations. For years, the standard for cavers has been three independent light sources of light. In a cavern, being without light basically means you can do nothing - the darkness is complete. Like many posters here, I also agree that redundancy in fire making often makes sense. Fire is critical and the means of ignition are rather light and easy to carry

I have usually seen the "one is none" routine applied to guns and knives, where it is just plain silly, seemingly an excuse to sell one more firearm and one more blade.

When I prepare for a backpack or a climb, I adopt the principle of "elegance" -I want critical and necessary items, but nothing in excess - the weight penalty is prohibitive. Multipurpose items are quite handy here. Sometimes the better strategy is not to simply pile on more gear, but have a plan
B involving improvisation and alternatives.