U(Kyou Machiko) summary:

Yuu has spent her entire life with a “copy” of herself, her twin sister. Now science has created a copy more perfect than nature ever could, and Yuu seems like the perfect test subject. But as her copy becomes more intelligent day by day and begins questioning its own existence, the experiment threatens to spiral out of control. Who is real and who is a copy? And what is a “copy” that has surpassed its original? Just a copy, the original, or something else entirely?