Well why not post my reply as well.
Nothing.
As simple as that, if it's purely physical clone I probably wouldn't even realize it, if it's personality as well, chances we meet are pretty low since I don't like to go out much. If we did meet, the one thing that I will probably feel strange, is that there is someone who knows where some of my stuff comes from, like my Sepulchritude shirt that no one I know so far know what it is.
As romance or not, well if it's physical and personality double, no. I have short hair, I prefer longer hairs on a girl, therefore she'd have short hairs as well, and would prefer longer hair, maybe in long term were we'd probably see each other as our opposite thanks to this very thread, we might not see ourselves as similar anyway since I doubt a true "opposite gender version of you" would keep a lot of similar physical traits at least, and we'd probably both assume we are in each others friend zone or something. So nothing more then inside jokes.