one useful point though is that my duplicates are NOT of the same item. I have duplicated some things because I have to have them (knife, firemaking and torch) but I dont carry two of the same model of anything. My backup isnt just in case one gets lost its also so that if something fails in a use due to a design problem or it just isnt suitable in those exact conditions them my backup will not fail in exactly the same way. (Lesson I learned for a father in law who designed the safety systems for nuclear power stations).

For example for firelighting I carry (spread over EDC, larger kit and BOB) a spark-lite, a magnesium block with ferrocerium striker, a windmill windproof lighter, a mini-bic lighter, a pack of wind and waterproof matches, and a film container with strike anywhere matches. Lots of redundancy but not two of anything.