A Prepper's Approach: Two Equals One and One Equals None

Have you ever gotten yourself into a challenging situation where you needed a tool and tried to use it? If the tool preformed well, that's great but oftentimes, we find that due to lack of testing or practice, the tool was either missing a part, needed an additional part, or wasn't working properly. 
Among the Prepper's community, we have the concept of Two Equals One and One Equals None. This is to ensure that in the absolute, worst case scenario when you absolutely needed something to bail you out, that object's integrity is guaranteed and would not fail you.

A good example is a water purifier. Many years ago, I purchased the Steripen a cool device that uses batteries and UV light to sterilize water. Out of curiosity, I tested it on tap water and tried drinking it. I then gagged and had an Allergic reaction. Sometimes, I would trust all the stellar reviews and not have a chance to test a product out. This time, I'm glad I did. If a challenging event occurred, and that was the only means I had to clean water, it would have been a great disaster. How about having multiple ways of obtaining clean water? There's chlorine, water pump's, gravity filter, and of course, boiling water. Having multiple methods of producing clean water would guarantee that we will have some means to have clean water. 

Another item to consider is heating food. There are many ways you can heat food and it would be a good idea to have all sorts of reliable tool on hand.