Duplicate NSNotificationCenter observer when segueing to subclassed custom UIViewController

Question

I have a custom UIViewController, called BoardViewController, and I have subclassed this to create GameBoardViewController. I added an NSNotificationCenter observer in viewDidLoad of BoardViewController. When I segue from another view controller to GameBoardViewController it seems this observer is duplicated somehow. Because now, whenever I cause that particular notification to be posted, the method called from the observer is called twice. I have tested making GameBoardViewController the initial view controller in the storyboard and there are no issues. It only happens when I segue to it.

Thanks for any help you can provide.

BoardViewController:

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

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(gameBoardTileWasTapped:) name:@"gameBoardTileWasTapped" object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(log:) name:nil object:nil];
}

GameBoardViewController:

- (void)viewDidLoad
{
    [super viewDidLoad];
// Do any additional setup after loading the view.
}