innerring: It's not a bug. There are two entries each - one for the Windows version and one for the Mac. ;)
It is a bug, I don't have a Mac, and of the 150 other games in my account none of the other ones that have Mac or Linux availability have multiple entries per platform showing up for platforms other than the one I am using. Additionally when you select one of them and the details come up for it, the details page is messed up with buttons missing and other glitches. There's no other way to categorize it other than "a bug".