“One is none and two is one” is an old saw for most serious preppers. It addresses the fact that if you have only one of something, you may soon have none since it is likely to break or wear out. Redundancy is important, but don’t forget to add some variety to your redundancy.
For example, imagine that you decided to count on a water filter to filter all your water. You go out and by a Katadyn Ceradyn filter or a Big Berkey. Knowing that “one is none and two is one” you buy the spare parts kit and some extra filters. Knowing that you can always boil water if necessary, you figure you are covered.