error CS0101: The namespace already contains a definition for 'X'

Read the Console line first

The message names the class, and the file and line point at one of the two declarations. Search the Project window for the class name to find the other.

Common causes

1. The same script was imported twice

   Downloading a script again or importing a package twice can leave PlayerController.cs and PlayerController 1.cs side by side. Both declare the same class.

2. An imported asset uses the same class name

   Asset Store packages and tutorials often use common names such as GameManager, AudioManager or Singleton. If yours has the same name in the global namespace, the two collide.

3. A backup copy is still inside Assets

   A copied folder such as Assets/Scripts_old is still compiled. Move backups outside the Assets folder or rely on version control instead.

The fix

namespace MyGame
{
    public class GameManager : MonoBehaviour
    {
        // Your class no longer collides with another GameManager
        // declared in the global namespace or in a different one.
    }
}

Stop it coming back

Put your own scripts in a namespace named after your game, and keep backups out of the Assets folder. When you import an asset, check whether it ships classes with the same names as yours before you write code against it.

Questions

How do I find the duplicate class quickly?

Search the Project window for the class name, or search the whole project folder in your IDE for class GameManager. Check Packages too: a local package can define it.

Can two classes share a name if they are in different folders?

Not in the same namespace. Folders do not separate code; namespaces and assembly definitions do. Two classes named GameManager can coexist in different namespaces.

Is it safe to delete the duplicate file?

Compare the two first: if they differ, keep the newer or the one your scenes use. Delete the copy together with its .meta file from inside Unity so the right GUID survives.

Scripts that avoid it

• Generic SingletonThread-safe, persistent singleton base class for managers like AudioManager, GameManager, etc.
• Audio ManagerComplete audio manager with music crossfade, SFX pooling, volume control, and persistence.

Related errors

• NullReferenceException: Object reference not set to an instance of an object
• MissingReferenceException: The object has been destroyed but you are still trying to access it
• IndexOutOfRangeException: Index was outside the bounds of the array
• ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection
• error CS0246: The type or namespace name could not be found

Got a different error? Paste it into the error lookup.