I just can’t resist redeeming them anyway. My Steam account is so full of junk but I like seeing that number go up. The only exception is if it’s something I’ll never ever have any chance of playing, a category that is really just fighting games.
For duplicate keys, I give them to people. I’m in a few small gaming communities where I can drop a key and it’ll go to someone who appreciates it. One in particular is very small but we gift substantial games to each other - I’ve got Starbound, Hearts of Iron 4, and Hollow Knight that way, and I’ve got to pay it forward eventually.
There’s also a thread on here for key giveaways.