I got one key which included the DLC shortly after release. Did you order from GMG? I heard some of Reddit got double keys.
@Grey, the DLC is included with the pre order, with ONE key.
On the Mad Max DLC info on Steam it should say installed/not installed. tick, download, restart game > garage > car bodies. You may have another copy of the game.
Yeah I finally got it some way in. I went to garage and it was available to unlock.
its a bug tony... game is still in 144hz!