I recently went through my PSK and got rid of a lot of stuff that duplicated what was in my EDC. If my PSK isn't real small I won't carry it, and if I duplicate I lose too much functionality. The main item I consciously kept redundant was light. That's partly because I figure it's important, and partly because there's a relatively high chance of failure. I did include several ways of starting a fire too, that being something my EDC is weak on. Apart from that it was mostly based on size, and whistles and compasses are just too bulky.