error CS0101: The namespace global: : ' already contains a definition for CameraController’
The problem is, there is already a class named CameraController.
Either do what nrgger says in their answer, or (in my opinion better) use a custom namespace. Write above the current line 7 something like
namespace Hassan.MyGame {
(You can name your namespace any way you want to, this is just a suggestion.) Don’t forget to close that bracket ({) at the end of the file.
IF you do this, you should do it in all your custom scripts. By doing it, your code is better ordered and protected from naming collisions like this.
If your project becomes more complex, you can always divide your namespace further.
More Info on namespaces might be interesting for you: C# Programming/Namespaces - Wikibooks, open books for an open world or Organizing types in namespaces | Microsoft Learn
Use a different name for you class at line 7, ‘CameraController’ is already used by another script.