Prevent duplicate Toast messages in Ionic

You can use a property on that page to know if a toast is being shown or not before showing a new one.

Ionic 2/3

import { ToastController, Toast } from 'ionic-angular';

// ...

private isToastVisible: boolean;

constructor(private toastCtrl: ToastController) { }

presentToast() {
  if(this.isToastVisible) {
    return;
  }

  this.isToastVisible = true;

  const toast: Toast = this.toastCtrl.create({
    message: 'User was added successfully',
    duration: 3000,
    position: 'top'
  });

  toast.onDidDismiss(() => {
    this.isToastVisible = false;
  });

  toast.present();
}

Ionic 4/5

import { ToastController } from '@ionic/angular';

// ...

private isToastVisible: boolean;

constructor(private toastCtrl: ToastController) { }

presentToast() {
  if(this.isToastVisible) {
    return;
  }

  this.isToastVisible = true;

  this.toastCtrl.create({
    message: 'User was added successfully',
    duration: 3000,
    position: 'top'
  }).then((toast: HTMLIonToastElement) => {

    toast.onDidDismiss().then(() => {
      this.isToastVisible = false;
    });

    toast.present();
  })      
}