NSTimer triggered twice after switching view controllers

I'm trying to keep accurate counting and display of a countdown timer between switching view controllers. In order to do so, I added NSNotifications as mentioned in this question: Timer Label not updated after switching views (swift)

The problem is that the timer will be decremented twice after switching back from the other view controller. This issue seems to be unrelated to the notifications, as the same problem occurs without them, it only becomes apparent after restarting the timer as it isn't updated automatically when reaching the view controller.

I really don't find the cause of this, any help much appreciated!

I've set up this sample code. There's another view controller added to the original one in Main.storyboard, there is one switch and one label displaying the timer added to it. The original view controller only contains one bar button item to trigger the segue to the second view controller.

import Foundation

final class DataModel: NSObject {

  static let shared = DataModel()

  var isSleepTimerOn = false
  var timerTime: NSTimeInterval = 100.0

}

// The second view controller.

import UIKit

class TimerViewController: UIViewController {

  @IBOutlet weak var timerLabel: UILabel!
  @IBOutlet weak var timerSwitch: UISwitch!

  var timer: NSTimer?

  override func viewDidLoad() {
    super.viewDidLoad()

    timerSwitch.on = DataModel.shared.isSleepTimerOn
    timerLabel.text = String(DataModel.shared.timerTime)

    let selector = #selector(setTimerLabel), name = "setTimerLabel"
    NSNotificationCenter.defaultCenter().addObserver(self, selector: selector, name: name, object: nil)
  }

  @IBAction func switchToggled(sender: AnyObject) {
    DataModel.shared.isSleepTimerOn = timerSwitch.on

    switch timerSwitch.on {
    case true:
      startTimer()
    case false:
      stopTimer()
    }
  }

  func startTimer() {
    let selector = #selector(decrementTimer)
    timer = NSTimer.scheduledTimerWithTimeInterval(1.0, target: self, selector: selector, userInfo: nil, repeats: true)
    NSRunLoop.mainRunLoop().addTimer(timer!, forMode: NSRunLoopCommonModes)
  }

  func decrementTimer() {
    DataModel.shared.timerTime -= 1
    NSNotificationCenter.defaultCenter().postNotificationName("setTimerLabel", object: nil)

    setTimerLabel()
  }

  func setTimerLabel() {
    timerLabel.text = String(DataModel.shared.timerTime)
  }

  func stopTimer() {
    timer?.invalidate()
    timer = nil

    DataModel.shared.timerTime = 100.0
    timerLabel.text = String(DataModel.shared.timerTime)

    NSNotificationCenter.defaultCenter().removeObserver(self, name: "setTimerLabel", object: nil)
  }

}