this seems odd

Illuminati at work?

probably

if you're sure they are dup, try to contact them.
If you are sure that they are regifting or doing something else than activating the key, revoke it and / or report it.

I hope I did understand what you meant. :#

This one is easy. if it is not marked as received, send a Request received feedback and include the account names, links, and screenshots of the same email addresses in the ticket. Support will be alerted and can handle it promptly.
I would not suggest a reroll ticket as with these kinds of users, you have a big enough chance that they are bots farming SG for freebie keys to sell.

Multi-accounting is unfortunately a component of SG, and heavily flooded on low-level public giveaways. If you think you've spotted it, you're probably right. Confronting the users isn't going to do any good- just stick all the relevant information into your support tickets and let support sort it out. There's nothing you can do on your end [except to up your giveaway entry level for future public giveaways, if you find that a desirable enough consideration].

don't worry, there are lots of people with alts on sg, from lv0 to lv10.
just report them and see if they can make a nice excuse so they can be allowed to keep leeching on sg like nothing happened.