Dupe key error = Region locked ?

If it said duplicated then it means the key was already used and has nothing to do with region lock.

Duplicate key = key was already redeemed by another account
Region locked = key can only activated in certain region(s)
Duplicate key =/= region locked

As a sec try, I gave my acc to one of my friends on another country but they failed to redeem it either.

It's against Steam's TOS to share your account data and also from an OpSec perspective I advise strongly against it.

My acc is global same as the giveaway's region but I suspect if it's been limited recently as everything was ok 1 month ago but this one........

I don't think that's how it works. Your "account region" is tied to the payment method you use. Eg. use a French bank account, your "account region" is European Union. There's no way to change it, lock it, or unlock it other than getting a new payment method in another country and I think requesting a change of location from Steam.

The only way region locks work is the key itself is region locked. When a developer/publisher creates the key, they can choose which regions they can be redeemed in. So keys that are region locked to Russia can only be redeemed in Russia, everywhere else you'll get a region lock error. Only keys that are ROW (rest of world) can be redeemed anywhere.

You got an error for a duplicate key. This means the key was already used and there is nothing you can do about it. Either the giveaway creator knew and tried to trick you into thinking there's some magical region lock, or he also doesn't know how it works and wasn't aware the key was used already, either by him or someone else.

The short is, key doesn't work. If the giveaway creator doesn't provide you a working key, you can either mark the giveaway as not received or agree to delete it and forget about it.

I also had the similar problem a few months ago. Got a 5$ gift card but couldn't redeem it even with vpn finally had to pass it to one of my friends to redeem on his own acc then giving it back by buying tf2 keys
As a sec try, I gave my acc to one of my friends on another country but they failed to redeem it either. Same error again

I'm guessing you're buying gift cards from grey markets? Physical gift cards are region locked to the country or area they're bought in. If a reseller on a grey market site buys his cards from a local store in Argentina and you live in Germany, you will not be able to redeem them. If the seller didn't disclose the key is locked, you kind of got scammed.