If I have a duplicate key in my inventory, can I give it to another player?
There's no way to securely trade items, but you can transfer them from one person to another by dropping them and having the other person pick it up. Obviously, this requires both of you to be in the same place.
Capsules are commonly used to transfer larger numbers of items in this way.
Yes you may by dropping the item onto the floor. The other player would need to pick it up from the same place. There is an expiration date (~24 hours) for items placed on the ground and it will disappear for good if not picked up.
You can drop any other items on the floor for other players to pick up. For larger amounts, I'd recommend using a capsule to drop up to 100 items to another player. You can get a capsule (Rare) from hacking.
You can not give the key to a specific person, but you can drop it at a physical location and arrange to have them pick it up.
My solution to this that I would like to try is creating and sharing a makeup ingress account. Here is how it would work.
You TRUST the person that you are trading with or gifting items.
Make a fake new Gmail and ingress account, choose either faction side.
log in to your real ingress account and drop on the ground the stuff you want to trade.
Log out on your phone of that account and into the "fake one". And pick up the items in to the inventory. Log out.
Give the log in credentials for that account to who ever it is that you want to have that stuff. Could be any one, anywhere in the world.
Now they log in to that same fake account with the log in credentials and drop the stuff you traded or gifted them on the ground in their area.
They log out of the fake account and into their real account and pick up the stuff on the ground.
Instant transfer of ingress stuff. Repeat for any purpose.
I've done this transfer between two of my own account and it works.
You can get keys or link amps etc. Rare items or one offs around the world fast this way.