Read this years ago on Manga Rock. Very messed up and didn't like the ending. Twisted (and not in a good way). The art is very pretty, though. Definitely not something 13 (?) yr old me should have read back then.
EDIT: WAIT, HOL' UP, WASN'T THIS IN BLACK'N'WHITE (+PINK) WHEN I FIRST READ IT?? DID I READ ANOTHER VERSION OF THIS OR WHAT?? HOW IS THIS IN COLOUR, HOLY HECK. WHAT.
EDIT 2: YEAH, MATE. THE ART IS KINDA DIFFERENT, TOO. WHAT THE HECK (the old art style looked better and fit in more with the whole "psychological" part of this )
EDIT 3: nvm, the old version is here as well
Vanishing Twin