I buy multiples for convenience and because I'm always forgetting things. I bought 2 extra power supplies for my netbook so I can have one at the office, one at home and the third in my travel bag. I was always forgetting to take the darn thing with me.
Because I have two work bags (one for the car and one for public transport, (size and weight differentials)) I also have two PSP's, multiple FAK's, flashlights, Ham HT's, etc. so I just have to pick up and go and not worry about moving stuff from one bag to another.
My dad was great at misplacing common tools and buying another one (usually blaming us kids for not putting them back where they belonged). When mom sold the house we found multitudes of the same tools scattered all over the place.
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In a crisis one does not rise to one's level of expectations but rather falls to one's level of training.