Meh, apparently been happening since after Adam and Eve... wait, technically Eve was made of Adam's rib so... does that mean she's a clone with an extra X?
Pretty much, yes. At least, that's how I always thought of it.
Wow, it gets even more disturbing when you attempt to apply science.
Sort of. Except that they both would have been genetically perfect, with no genetic flaws or anything, thus reproducing between both of them would have produced viable and ultra healthy offspring.
The "gross factor" of a brother and sister reproducing is purely cultural, as some (though certainly the vast minority) of cultures allowed and even encouraged incestual relationships.
But they were also the FIRST FREAKING HUMANS... so it's just different. lol.
EDIT: hypothetically speaking, of course. :p