I'm with you, OP; some things are important enough to accept the weight penalty and take two.
1) Headlamps. I take an extra battery anyway. An 18650 battery needs to be stored in a case. I use a second Zebralight headlamp as my "battery case", as it only weighs 1.4 oz.
2) Knives. I take one fixed blade and one Havalon
3) Fire starters. Lighter and scraper.
4) Socks! Been burned with wet socks, plus I sleep in a clean pair anyway to keep my bag clean
5) Chemical foot warmers. I always take two pairs for comfort or emergency.
6) Shelter systems. I carry a Zpacks Pocket Tarp, which is a complete shelter. 5.3 oz. This is in case I get caught out overnight or in some crazy storm.