I don't carry a lot of "backup" preferring instead to take stuff that doesn't break

I carry two mini-bic lighters. one of those is definately a backup.

I do carry two sets of socks, but one is for sleeping, and one for hiking

I take CL02 solution, not a filter, so I don't really need a backup for that

When carrying a thermarest, I carry a small patch kit.

Beyond that, there's really not a lot of stuff that I carry that has much a chance of breaking - and if I were to break it in any real way, I'd be uncomfortable but not in any real danger.