Progressive Web App - Fetch request for post is posting data twice

Question

I am facing the issue that when I am posting a data using fetch API, I am getting the data posted twice. my service-worker code code is like this:

self.addEventListener('fetch', function(event) {
  if(event.request.method === "POST" || event.request.method === "PUT" || event.request.method ==="CREATE" ){
   return fetch(event.request);
  };
  if(event.request.method === "GET"){
  event.respondWith(    
    caches.open(cacheName)
       .then(function(cache) {
      return cache.match(event.request).then(function (response) {
        return response || fetch(event.request).then(function(response) {
         // cache.put(event.request, response.clone());
          return response;
        });
      });
    })
  );
  }
}) 

and JS file code is like this:

let leaveRequestForm = document.getElementById('leave-request');
    leaveRequestForm.addEventListener('submit',function (e){
    e.preventDefault();

let formData = new FormData(leaveRequestForm);

fetch("https://myserver/api/leave-request/create-leave-request", { 

    method: 'post', 
    "headers": {        
        'id':id,            
    },        
    body: formData
}).then(function (res) { 

    if (res.status >= 200 && res.status < 400) {             
        var successMsg = "<div class='alert-success'>Your Request is Submitted Successfully.  <br>Thanks</div>";
        var target = _d.qs(".content-target");
        target.innerHTML = successMsg;

   return res.json(); 
    } else { 
  console.log('Status Code: ' + response.status);      
     return; 
    } 
}).then(function (resp) { 
    //process the response 
})
});

What I am missing here? I can comment-out the code in service-worker return fetch(event.request);, but I want to send the request through service worker as I plan to offlice submit as well