Summer: backup lighter - one in the stove bag, one in the ditty bag. That's it - I do carry three pair of socks, but I don't consider that backup, since I wash a pair each day. I guess you could say one pair is backup - one to wear while the other dries and the third is backup in case the one doesn't dry that day.

multi-day ski trips: I carry a photon micro light as backup to my headlamp; a second pair of liner gloves (lost one once, and can't be without); and backup sunglasses - at 11 or 12K on the snow in early May, you gotta have shades!