I like the idea of this story. Again, you are good at writing about Riku's mental illness. And the concept of the double wanmting to be the real Riku is excellent, as is Riku's dispassionate cataloguing of his own weaknesses and strengths as he fights his shadow.
Sometimes, I think the reactions of the shadow and of Riku were a bit too much. For instance, I like that Riku kissed his shadow, but when he ends up having sex with it, I feel like that may be going just a bit too far. He's a quite young man and don't you think that despite the fact the shadow is only him, it might still be a little horrific to engage in intimacy with it? Obviously it's really a matter of taste, but I felt like I would have enjoyed the story more in Riku had a different reaction to the kiss he gives his double.
Another thing that struck me is the swearing. It is believable, but I wondered if you could have thought of other ways to express Riku's disgust with himself and with Sora. Fr instance, when Riku says "you're slow" to the fake and the fake repeats, brokenly, "shut up," I found that much more effective dialogue than the swearing.
Also the reference to Namine (I had to look it up because I haven't played Chain of Memories) seems out of place here unless you are planning a sequel to this story.
But back to the things that really work in this story, once again I love the way you write Riku. You have a real insight into his character. On the one hand he's thinking of hugging his mother, on the other hand he's fighting a twisted shadow version of himself. You write Riku's madness so much more effectively than the game portrays it.
There are just so many excellent insightful moments in this story, as when Riku realizes the fake will actually kill him because it wants to be him. I think that with some editing, just editing to take out things that don't seem to fit in this story and tightening up of details would make this story even better than it is now.
I am hoping you'll write more Kingdom Hearts fiction, especially fiction with Riku in it. I really like the way you write him.