Ionic 4 back button callback method executing twice

I am using ionic platform backbutton to over ride the hardware back button functionality. Its working fine to load the previous page but its calling the callback method of backbutton subscribeWithPriority twice. So if i display the pop up in callback method, that pop up is displaying twice which is very annoying. Below is the code i have added in constructor of my app module.

 this.backBtnSubscriber = this.platform.backButton
        .subscribeWithPriority(9999,
          () => {
            this.alertDialogService.presentAlert();
          });

Below are details of ionic info:

Ionic:

   ionic (Ionic CLI)             : 4.3.1 
   Ionic Framework               : @ionic/angular 4.0.0-rc.0
   @angular-devkit/build-angular : 0.7.2
   @angular-devkit/schematics    : 0.7.2
   @angular/cli                  : 6.1.1
   @ionic/angular-toolkit        : 1.2.0

Cordova:

   cordova (Cordova CLI) : 8.1.2 (cordova-lib@8.1.1)
   Cordova Platforms     : android 7.0.0, ios 4.5.5
   Cordova Plugins       : cordova-plugin-ionic-keyboard 2.1.3, cordova-plugin-ionic-webview 2.3.1, (and 15 other plugins)

System:

   Android SDK Tools : 26.1.1 
   NodeJS            : v10.4.1 
   npm               : 6.1.0
   OS                : Windows 10