I really wanted to like this book, as I do enjoy a lot of children's and young people's fiction, but I found it very frustrating.
It was very readable, and it did draw you in well. However, I got frustrated by areas where the science or setting or plot was implausible or just plain wrong.
I found that the characters didn't really act like people would. People were just too inclined to help this random kid whenever he showed up - it didn't feel real.
I don't know much about the science behind birthmarks. But I DO know that being genetically identical to someone DOES NOT mean you will have identical birthmarks. That is just gibberish. You may have a high likelihood of both having the same type of birthmark, but not The Same Shaped One In The Same Place. Surely a single person with GCSE biology checking the manuscript could have explained this....
I couldn't get past the implausibility of the whole world suddenly deciding together that they were going to reintroduce the death penalty for a single crime, and have that crime be the cloning of humans. Presumably they applied the law retrospectively too, since the cloning in the book would have taken place in 2001, when we know that such a global law didn't exist (this book was written after then). Perhaps if all the handwringing over that had been taken out, then we could have actually explored the feelings the boy mentioned occasionally, rather than glossing over the questions he asked.
There were other similar problems, but these were the main ones. I'm glad I read this book, and I did enjoy the story, but it did make me want to shout repeatedly at the author. Which is a shame, because it could have been awesome.