Ionic alert controller is calling twice when function is invoked via handler in button

Ionic Framework ionic-v3
1

import { Component } from ‘@angular/core’;
import { IonicPage, NavController, NavParams, Platform, AlertController} from ‘ionic-angular’;
import { Dialogs } from ‘@ionic-native/dialogs’;
import { CodePush , SyncStatus, InstallMode } from ‘@ionic-native/code-push’;

@IonicPage()
@Component({
selector: ‘page-welcome’,
templateUrl: ‘welcome.html’,
})

export class WelcomePage {

private appCurrentVersion;

constructor(public navCtrl: NavController, public navParams: NavParams,private codePush: CodePush,private dialogs: Dialogs,public alertCtrl:AlertController,public platform:Platform) {
	this.platform.ready().then(() =>{
		this.codePush.getCurrentPackage().then(data => {
			this.appCurrentVersion = data.appVersion;
		})
		.catch(e => console.log('Error displaying dialog', e));
		this.codePush.notifyApplicationReady();
	});
}

ionViewDidLoad() {
	console.log('ionViewDidLoad WelcomePage');
	this.codePush.checkForUpdate().then((data)=>{
		if(!data){
			console.log("App is UP-TO-DATE");
		}
		else{
			this.codePush.sync({installMode: InstallMode.ON_NEXT_RESUME});
		}
	})
}

showAlertMessage(event){
	const alert = this.alertCtrl.create({
        title: 'About Antibiotic Policy',
        message: `
			Current App Version is ` + " " + this.appCurrentVersion + `<br>
			<br>
			<span> Developed by </span> <br>
			<br>
			<strong> Raster Images Pvt Ltd </strong>
			`
        ,
		buttons: [
			{
				text : 'CHECK UPDATE',
				handler : () => {
					this.update();
				}
			},
			{
				text : 'Cancel',
				role:'cancel'
			}
		]
      });
	alert.present();
}

update(){
	this.platform.ready().then(
		() => {
			this.codePush.sync({updateDialog: true,installMode: InstallMode.ON_NEXT_RESUME}).subscribe((status) => {
				if(status == SyncStatus.CHECKING_FOR_UPDATE)
					this.dialogs.alert("Checking for update");
				if(status == SyncStatus.DOWNLOADING_PACKAGE)
					this.dialogs.alert("Downloading Package");
				if(status == SyncStatus.IN_PROGRESS)
					this.dialogs.alert("In Progress");
				if(status == SyncStatus.INSTALLING_UPDATE)
					this.dialogs.alert("Installing Update");
				if(status == SyncStatus.UP_TO_DATE)
					this.dialogs.alert("Up to Date");
				if(status == SyncStatus.UPDATE_INSTALLED){
					this.dialogs.alert("Update Installed");
					if(SyncStatus.UPDATE_INSTALLED){
						this.codePush.restartApplication();
					}
				}
				if(status == SyncStatus.ERROR)
					this.dialogs.alert("Error -> Check Internet Connection");
			})
		}
	)
}