确保添加一次观察者

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];

    [[NSNotificationCenter defaultCenter] removeObserver:self name:kCESHITONGZHI object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];

    [[NSNotificationCenter defaultCenter] removeObserver:self name:kCESHITONGZHI object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];

    [[NSNotificationCenter defaultCenter] removeObserver:self name:kCESHITONGZHI object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];
    
}

- (IBAction)btnClick:(id)sender {
    
    [[NSNotificationCenter defaultCenter] postNotificationName:kCESHITONGZHI object:nil userInfo:nil];
    
}


-(void)showMSG{
       NSLog(@"I'm here");
}

执行结果：
测试重复通知[7866:2161604] I'm here

多次添加观察者

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
   
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(showMSG) name:kCESHITONGZHI object:nil];
    
}

- (IBAction)btnClick:(id)sender {
    
    [[NSNotificationCenter defaultCenter] postNotificationName:kCESHITONGZHI object:nil userInfo:nil];
    
}


-(void)showMSG{
       NSLog(@"I'm here");
}
    
}

- (IBAction)btnClick:(id)sender {
    
    [[NSNotificationCenter defaultCenter] postNotificationName:kCESHITONGZHI object:nil userInfo:nil];
    
}


-(void)showMSG{
       NSLog(@"I'm here");
}

执行结果：
测试重复通知[7869:2162423] I'm here
测试重复通知[7869:2162423] I'm here
测试重复通知[7869:2162423] I'm here
测试重复通知[7869:2162423] I'm here