mystere = int(random(1,101)) ; //tirage d'un entier entre 0 et 100 NON INCLUS
Your comment is wrong. The random() function, with those arguments does not return a value between 0 and 100.
What is sending data to the Arduino? If it is the Serial Monitor, what line ending setting (lower, right corner) are you using?
Edit: Never mind. I looked at your picture. It appears that the Serial Monitor is adding a carriage return to the string entered. Change that to nothing appended to the string to get rid of the duplicate response.