I got that too recently. From googling it seems that happens when someone tries to register a key that you already registered to your account. Like the free keys people post in threads.Hey guys, I just received this email from Steam. Should I be worried?
Edit: links to threads about this.
http://www.steamgifts.com/forum/7qAJu/i-need-to-be-feared
http://forums.steampowered.com/forums/showthread.php?t=2665303