observeValueForKeyPathが二度呼ばれてしまう問題...

CallTwiceObserveProblem

#import "AppDelegate.h"

@interface AppDelegate ()
@property (weak) IBOutlet NSWindow *window;
@end

@implementation AppDelegate
    
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    NSUserDefaults *userDefault = [NSUserDefaults standardUserDefaults];
    
    // Remove All Preferences settings.
    [userDefault removePersistentDomainForName: [[NSBundle mainBundle] bundleIdentifier]];
    
    // Set Default value to UserDefaults
    [userDefault registerDefaults:@{@"test":@NO,}];
    
    // Observing "testValue"
    [userDefault addObserver:self
                  forKeyPath:@"testValue"
                     options:NSKeyValueObservingOptionNew
                     context:nil];
    
    // after change, call observeValueForKey...
    [userDefault setBool:YES forKey:@"testValue"];
}
    
    
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"Why call twice!?!?");

}
// 2018-01-22 20:55:43.658261+0900 KVOTestForObjC[22625:762757] Why call twice!?!?
// 2018-01-22 20:55:43.658357+0900 KVOTestForObjC[22625:762757] Why call twice!?!?
    
    
@end

NSUserDefaultの値をKVOで見張り、値を変更すると何故か二度変更の通知が飛んできてしまう問題。
何が原因だか今のところ不明。