error CS0101: The namespace `global::' already contains a definition for `GameManager'

hello Im very new to the whole scripting and unity engine . When importing project , I have this error

error CS0101: The namespace global::' already contains a definition for GameManager'

When I click the error appears to me :

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class GameManager : MonoBehaviour {
     [Header("Setup Level")]
     [Tooltip("Is this final level of the World? the next World will be unlock")]
     public bool isFinishWorld;
 
     public int bullets = 10;        //bullets for player fire
     public int star1 = 100;            //score needed to reach 1 star
     public int star2 = 500;            //score needed to reach 2 star
     public int star3 = 1000;        //score needed to reach 3 star
     [Header("")]
     public AudioClip soundSuccess;    //sound played when level complete
     public AudioClip soundFail;        //sound played when fail
 
     public static GameManager instance;        //global call
 
     public enum GameState
     {
         Menu, Playing, Pause
     }
     [HideInInspector]
     public GameState state;
 
     private int score = 0;
     private int stars = 0;
     private int heart = 5;
 
     //Get Score from another script ex: int score = GameManager.Score;
     public static int Score{
         get{ return instance.score; }
         set{ instance.score = value; }
     }
 
     //Get Stars from another script
     public static int Stars{
         get{ return instance.stars; }
         set{ instance.stars = value; }
     }
 
     //Get Hearts from another script
     public static int Hearts{
         get{ return instance.heart; }
         set{ instance.heart = Mathf.Clamp(value,0,7); }
     }
 
     //Get Bullets from another script
     public static int Bullets{
         get{ return instance.bullets; }
         set{ instance.bullets = value; }
     }
 
     //Get Best of this level from another script
     public static int Best {
         get{ return PlayerPrefs.GetInt ("World" + GlobalValue.worldPlaying + GlobalValue.levelPlaying+"best", 0); }
         set{ PlayerPrefs.SetInt ("World" + GlobalValue.worldPlaying + GlobalValue.levelPlaying+"best", value); }
     }
 
     //Get BestStars of this level from another script
     public static int BestStars {
         get{ return PlayerPrefs.GetInt ("World" + GlobalValue.worldPlaying + GlobalValue.levelPlaying+"stars", 0); }
         set{ PlayerPrefs.SetInt ("World" + GlobalValue.worldPlaying + GlobalValue.levelPlaying+"stars", value); }
     }
 
     //Get Highest Level of this level from another script
     public static int HighestLevel {
         get{ return PlayerPrefs.GetInt ("World" + GlobalValue.worldPlaying + "HighestLevel", 1); }
         set{ PlayerPrefs.SetInt ("World" + GlobalValue.worldPlaying + "HighestLevel", value); }
     }
 
     //Get current state from another script
     public static GameState CurrentState{
         get{ return instance.state; }
         set{ instance.state = value; }
     }
         
     private PlayerController player;
 
     void Awake(){
         instance = this;
     }
 
     // Use this for initialization
     void Start () {
         state = GameState.Menu;
         player = FindObjectOfType<PlayerController> ();
 //        AdsController.HideAds();
     }
 
     // Update is called once per frame
     void Update () {
         
         if (Input.anyKeyDown && state != GameState.Playing)        //Start playing when hit anykey or any touch
             Play ();
     }
 
     //play game
     public void Play(){
         state = GameState.Playing;        //set state to playing
         player.Play ();        //call funstion Play() in player object
     }
 
     //called by another script by: GameManager.instance.GameSucess();
     public void GameSuccess(){
 //        AdsController.ShowAds();        //show admob
 
 //        if (ApplovinController.Instance != null)
 //            ApplovinController.Instance.ShowAds ();        //show applovin
 
         state = GameState.Menu;        //set state to Menu to stop player, enemy, monster,...because they only work when state = playing
         //check level pass
         if (GlobalValue.levelPlaying >= HighestLevel)
             HighestLevel = GlobalValue.levelPlaying + 1;    //if this level >= highest level then save next level number
 
         //check and save best score
         if (score > Best) {
             Best = score;
         
         //save best star of this level
             if (score >= star3 && BestStars < 3)
                 BestStars = 3;
             else if (score >= star2 && BestStars < 2)
                 BestStars = 2;
             else if (score >= star1 && BestStars < 1)
                 BestStars = 1;
         }
         MenuManager.instance.ShowLevelComplete ();        //Tell UI object show level complete panel
         SoundManager.PlaySfx (soundSuccess);        //play success sound
 
         //Unlock New World
         if (isFinishWorld) {
             int WorldReached = PlayerPrefs.GetInt ("WorldReached", 1);
             if (WorldReached == GlobalValue.worldPlaying)
                 PlayerPrefs.SetInt ("WorldReached", GlobalValue.worldPlaying + 1);
         }
     }
 
     //called by another script by: GameManager.instance.GameOver();
     public void GameOver(){
 
 
         if (state == GameState.Playing) {    //only work when in playing mode, to prevent duplicate calling
             state = GameState.Menu;        //set state to Menu to stop player, enemy, monster,...because they only work when state = playing
 //            MenuManager.instance.ShowGameOver ();    //Tell UI object show Game Over panel
             StartCoroutine(Restartcheckpoint(2));
             SoundManager.PlaySfx (soundFail, 0.5f);        //play fail sound
             player.Dead ();        //Tell player operate Dead() function
 
 //            AdsController.ShowAds();        //show admob
 
             //        if (ApplovinController.Instance != null)
             //            ApplovinController.Instance.ShowAds ();        //show applovin
         }
     }
 
     IEnumerator Restartcheckpoint(float time){
 
         yield return new WaitForSeconds (time);
         SceneManager.LoadScene (SceneManager.GetActiveScene ().buildIndex);
     }
 }
 

please help me