I don't carry much by way of backups: a sparklite with tinder in case my stove doesn't work, some iodine tablets as backup for my filter or to make a wound cleansing solution in a pinch, duct tape if something rips, 20' of cord for repairs or for lowering my pack when scrambling gets too tough. I used to carry some nonlubricated condoms as backup canteens, but felt silly about it seeing them in my bag after a few trips.

Arguably, the foam pad in my daypack could be used to sleep on. I carry "backup"-grade essentials (button compass, zipper light, etc.) for easy dayhikes where nice equipment feels like overkill.

Oh, I do carry backup prescription glasses. My eyes are probably legally blind by themselves. I picked up a pair of folding reader glasses & had some lenses installed. They pack away tiny.