UserDefaults.standard.addObserver calling observeValueForKeyPath multiple times

KVOProblem.swift

import Cocoa

let testKey: String = "TESTKEY"
let defaults: UserDefaults = UserDefaults.standard

// Remove Perferences & register default
defaults.removePersistentDomain(forName: Bundle.main.bundleIdentifier!)
defaults.register(defaults: [testKey : false])

class testClass: NSObject {
    var callCount: Int = 0
    override init() {
        super.init()
        defaults.addObserver(self, forKeyPath: testKey, options: .new, context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        self.callCount += 1
        print("Hello World: (\(self.callCount)times)")
    }
}

let testObj = testClass()
defaults.set(true, forKey: testKey)

code executed by Swift Playground.

Result:

Hello World: (1times)
Hello World: (2times)

Why observeValue call twice??

macOS: High Serra 10.13.2（17C205)
Xcode: Version 9.2 (9C40b)

Seems to work ok now in Xcode Version 11.4 beta 3 (11N132i).

import UIKit

let testKey: String = "TESTKEY"
let ud: UserDefaults = UserDefaults.standard

// remove preferences & register default
ud.removePersistentDomain(forName: Bundle.main.bundleIdentifier!)
ud.register(defaults: [testKey : false])

class testClass: NSObject {

    var callCount: Int = 0

    override init() {
        super.init()

        ud.addObserver(self,
                       forKeyPath: testKey,
                       options: .new,
                       context: nil)
    }
    
    override func observeValue(forKeyPath keyPath: String?,
                               of object: Any?,
                               change: [NSKeyValueChangeKey : Any]?,
                               context: UnsafeMutableRawPointer?) {
        self.callCount += 1
        print("Hello World: (\(self.callCount) times)")
    }
}

let testObj = testClass()
ud.set(true, forKey: testKey)

Hello World: (1 times)

Thx for Replying!