How far do you take it? Should I carry a second Iphone? A second Rolex? We all carry an extra knife. I carry 2-3 lighters and 3 credit cards. Silver rings for emergency currency. If I could carry a gun, I'd carry 2-3. Body armor is one of those things where you can't carry more than 1 practically.
What do you carry with redundancy?
I like redundancy - 2 is 1, 1 is none is very true if something breaks.
I always carry more than 1 flashlight - I EDC 3. None of them weigh a lot, but they all work well. I like to be able to see, even if it is dark - vision is important to me.
I always carry more than 1 knife - I EDC a knife + LM + SAK. They have different functions, but they can back each other up to some extent.
I EDC one lighter, but if I am going camping I take multiple fire starting tools - several lighters & a firesteel.
I have a watch and as a backup the cellphone can tell me the time - so I guess I have redundancy on time telling gadgets. Since its function is not critical I have never considered carrying more than one watch.
I always keep 2 hankies on me - if I have a cold and blow my nose on one then I still have a clean one in case I cut myself or whatever.
I work as a computer tech and frequently use USB flash drives, I carry 3 x 8GB drives just in case one fails or gets lost.
Here we can't carry firearms. If I lived in the US and had a CCW permit then I might consider carrying a small backup. If I was going hunting with a rifle then I wouldn't bother with a spare firearm - if my rifle failed that wouldn't be critical as it isn't an essential survival item. Obviously a rifle failure would mean an end to hunting though.
Body Armour - I am not overly familiar with it, I don't really know why you might need a spare. I presume this is more useful in dangerous urban or combat situations (not so much for camping or hunting) - if you were working as law enforcement in a very dangerous city then you could carry spare body armour in the vehicle I suppose.