Y/A (?) dystopian or sci-fi book

Hello! I'm looking for the name of a book, I'm not 100% on some of the details but here's what I can remember:

Fiction
Plot: A group of children is kept in the woods (?) where they are trained on one planet so that they can go through this portal in the middle of a body of water. Their planet is dying and they are trained in order to kill their double on the other copy of Earth and take over their life. I believe one of the main protagonists was French/her double was French.
When: I read this book around 2017 and I think it was published in the 2010s.
Audience: Y/A
Other: The book was very thick and hardcover, the dust jacket was a light blue-ish grey with black on it.

Thanks in advance.

The Space Between Worlds by Micaiah Johnson?

>2 juels: I don't think that's it, unfortunately