Push notifications are showing twice in service worker using Firebase

I am getting notifications twice when the application in the background. Here is my Service-worker file. When we call this firebase.messaging() function, the service-worker starts receiving and displaying push notifications automatically. Service-worker is showing both notifications. one initializing by the firebase and one with this code self.registration.showNotification(notificationTitle, notificationOptions).

  importScripts('https://www.gstatic.com/firebasejs/8.2.3/firebase-app.js');
  importScripts('https://www.gstatic.com/firebasejs/8.2.3/firebase-messaging.js');

  // Initialize the Firebase app in the service worker by passing in
  firebase.initializeApp({
    apiKey: "apiKey",
    authDomain: "authDomain",
    projectId: "projectId",
    storageBucket: "storageBucket",
    messagingSenderId: "messagingSenderId",
    appId: "appId"
  
  });

  const messaging = firebase.messaging();

  messaging.onBackgroundMessage( function(payload){
   
    const notificationTitle = payload.notification.title;
    const notificationOptions = {
      body: 'Testing',
    };

    return self.registration.showNotification(notificationTitle, notificationOptions);

  });

How to prevent default notification?