I always carry back-up everything in multiple places, especially knives. I have them everywhere :thumbup:
I buy two or three of things for that reason , and sometimes if I just like something a lot, or maybe it's an irreplaceable item in the future. Usually applies to knives though
I generally only buy multiple of one particular item if it is cheap - I have quite a few lighters for example.
For flashlights I do have several similar lights - I have a few 1 x AA powered LED lights for example. These are great because they utilize the best possible battery format and the lights themselves are a perfect combination of high output & small size & weight. AA batteries must be the most common size out there and I have plenty of rechargeable and non-rechargeable cells in that size. If I go camping with 2 of these lights then each one holds a spare battery for the other if it is needed - one light fails and then I use up the battery from the other, no problem I can take the battery from the dead light and put it in the good light.
For knives I have a bunch of different ones for different tasks, but for any given task I have more than one that would do that task well. Sure my BK-7 can't chop as well as my BK-9 does - but in an emergency I can chop & baton with the BK-7 if I have to. My stainless Mora is my food prep knife, but I can also use my carbon Mora or my Opinels or my Endura 4 or my RAT-1. My SAK is a Victorinox Cybertool and many of its functions are interchangeable with my Leatherman Wave - I EDC both and some things are definitely easier to do with one or the other. But between my SAK & my LM Wave I have: 4 blades (3 plain, 1 serrated), 2 pliers, 2 scissors, 2 can openers, 2 bottle openers, 2 of each type of screwdriver (well - flat & phillips anyway). OK so I only have one corkscrew, 1 saw & 1 file - but I'm sure I could make do. I'd rather add a different SAK than buy another one of what I have - this adds versatility while covering some of the more important features for redundancy.
I have a BK-9 and a KA-BAR Kukri Machete and on the way is an HI Ganga RAM Khukuri. That is 3 choppers, they are all different in size, weight and shape - but they are all capable of chopping & batoning and they give me both versatility & redundancy. I wouldn't buy 3 of the BK-9, but I like that I have 3 different knives that can cover each others tasks if required. I might not take all 3 camping, but if weight and space aren't at a big premium then I might take the KA-BAR or HI as a chopper and my BK-9 (or at least BK-7) as a backup.
I have 4 Opinels: #4, #6, #8 stainless & #9 - they didn't cost much but they give me 4 different sizes to choose from and they can also cover each others tasks to some degree. For many tasks the #8 or #9 would work well and the #6 would be able to do the job if it had to.
I like to be well equipped with the right tool for the job and at least one backup tool that could also handle the job if needed. It makes me feel like I am well prepared for whatever.
If I was camping with someone else and we were working together to prepare food or whatever then I would have a spare knife that they could use if required. This wouldn't normally happen - they would have their own knife, but in an emergency survival situation you may need someones help and you may need to equip them to help you. If someone is lost at night and you need to search for them it could be good to have someone else help in the search - I could give the other person a flashlight to use while I use another one, 2 is better than one in that situation! (and I would still have a spare light because I EDC 3 of them)