When I start a new piece of music, a twin version is created automaticly

Are you talking about the "navigator" - the tiny thumbnail representation of your score at the bottom of the screen? That is there to help you navigate, as the name implies. By dragging the blue rectangle around, you can quickly move to different places in your score. But it is not the only way to navigate - dragging the score with the mouse, page up/down, and the mouse scroll wheel all make it possible to move around without the navigator. So if you prefer not to have that method of navigation available, simply go to the Display menu and turn it off. But there should normally be no need to do that, and it can be quite useful. Why do you say you "have to" move it in order to proceed?

Oh, maybe you're not talking about the navigator - maybe you have inadvertently turned on the "Documents Side by Side" or "Documents Stacked" options, also in the Display menu? Those are there to let you view multiple files at once, but serve no useful purpose if you are only editing one document, you should turn those options back off.