Not sure what to do

Doing giveaways for groups is a great way to giveaway that possible duped keys, since users will have to be fair with your "maybe dupped" warning, I mostly do giveaways for groups /whitelist only and when I'm not sure if a key is dupped and don't have an extra key(sometimes I end buying a second one when this happens), 100% people has agreed to delete the giveaway when the dupe key happens.

Doing it as a public giveaway will probably end in a non received key (if the key is dupped), because of the winner having to agree, so thats why I'm a group man atm.

You can make invite only givs here with the understanding that if they are dups people should agree to delete them, it's been done before.

use a group like this if you are unsure.

https://www.steamgifts.com/discussion/s1tVq/ninja-retreat-a-group-for-giveaways-of-once-free-games

The group allows a certain number of keys to be given away and if they are dupes, the winner by the rules must allow you to delete the giveaway or try to resolve it first.

Nothing to add to the subject, but thanks for the giveaway!

Probably best to make group and/or invite-only GAs where you could replace a non-working key if cheap enough or delete the GA. For regular GAs people are not obliged to let you delete the GA so this is too risky.

Hey there, fellow railroader! Have a bumper

if I understand correctly once you could do group GA with special rules as deletion in case of bad key, but afaik it is no longer allowed, even if people still do it (so it's probably tolerated, faq maybe helps).
if you come here every now and then it means this is not your top priority, so I guess both forum drop (no risk and quick, but at least with some ninja/bot protection) and group ga (some small risk and less quick) will do the trick.

Use this everytime you are ot sure/frustrated and so on

Group or invite only giveaways should do the job like others here said.
Thank you for the giveaway!

You can also dump the keys on the orphan keys thread.

others proposed the best option I thinkg, group GA, where people would accept deleting GA if key is not working
and thanks a ton for a great GA :)

Just made GA for your trust whitelist :)

If you want to do a key dump in the forum, you could always put some rules if you don't want them to get regifted.
You could for exemple give it to people who have it on their wishlist.

Nothing to add except a +1 for Ninja Retreat 🐱‍👤
    ...but thank you for the giveaway! 😊

Another +1 for Ninja Retreat.

I'm going to go totally off topic here and ask what it's like working for a railway? That sounds awesome.

I would use my whitelist in this case, I am sure that most of the people there would understand the situation :)

HB gift links sometimes have issues if you leave them unclaimed too long after generating them. It's best to always wait to generate them until the point where you'll actually be utilizing them; Humble shouldn't basically ever be giving you a non-functional key outside of that specific circumstance.
It may be possible to get Humble to reset the gift links for you [since it's a known issue] if you're sure of which ones shouldn't have been used, but do be careful not to indicate that you plan on utilizing them for any purpose outside of self-activation or directly gifting to friends. Humble doesn't seem to have the best outlook on SG or trading sites, these days, seemingly being unable to differentiate such sites from for-profit-oriented trading/grey market sites.

You'll have overwhelmingly high risk of issues when asking for respectful interactions on public giveaways or open group giveaways, low risk with invite giveaways where you clarify the context beforehand, lower risk with SGT-filtered invite giveaways where the context is clarified, and all-but no risk when utilizing any specialized or generally respectable SG group.
The risk with key drops is that exploiters will nab them for card-farming; Regifting is going to be impossible in any circumstance where a key can be viewed by multiple users, and by having the individual on your friends list you can confirm that any directly given keys are activated as they're received [and if the user fails to activate, simply keydrop the key to prevent regifting] by checking the game's store page for 'Friends that own the game'.

There's not really any way around the risk of any given-away key, regardless of gifting method, being used wastefully- however, you can utilize an SGT filter to filter the gifting to specific characteristics [ie, the user having the game wishlisted and having a small wishlist size] to mimize risk. It's easy enough to then get the filtered users into a group made for the giveaways, or to gift the games directly by way of utilizing a random number generator as an alternative winner assignment method.