i guess he's not that dumb to giveaway this

He can't purchase another copy to giveaway... but he may already have a key for it, or have a gift in his inventory (Privacy settings could be set to "Keep your Steam Gift inventory private, regardless of your inventory setting above.")... I assume he's read the rules for this Community Guidelines for this website, and is giving away a legit copy of DiRT Rally that he has available right now, so that when this giveaway ends in about 23 hours, the winner will get exactly what is expected

Since you did a half-assed job of playing detective, I decided to finish the job for you. He has DiRT Rally in his library, but only has .6 hours played and 3 achievements. All 3 achievements were unlocked on December 30th 2016. That is also the same date of the game's lowest actual price on Steam. He also has Inside in his library. That game has 3.6 hours played and 4 achievements unlocked on April 22nd and 23rd of this year. That's about the same time that Humble Monthly for May released both of those games. The safe bet is that he's a subscriber and is offering this game up since he already owns it and has already paid for it.
I could be totally wrong tho lol

I did reserach too, always do that when so pricey game is on the giveaway...
This one seems legit to me.