title: Fullcalendar is duplicating event on drag and drop
body: <p>My fullcalendar duplicates events visually when i drag them to another timeslot. I have simplified my code down to the eventDrop to isolate the issue and yet I'm unable to understand the issue.
  If I store the events to my localStorage I don't get a duplicate in the storage and the duplicate disappears when I reload the page. This means the problem is only visual and with Full Calendar itself.
  However, this is obviously a huge issue as I don't want to reload the page: I want to stay in the current view changing what I need.</p>
  
  <p>Here's my code for the eventDrop:</p>
  
  <pre><code>eventDrop: function(event, delta, revertFunc, jsEvent, ui, view) {
              if (!confirm("Are you sure you want to change " + event.title + " ?")) {
                      /*If the user cancels the change, the event returns to its original position. Otherwise it saves the event.*/
  
                      revertFunc();       /*Revert changes using the predefined function revertFunc of fullCalendar.*/
  
                      $("#calendar").fullCalendar("refetchEvents");
                      /*FullCalendar Method to refetch events from all sources and rerender them on the screen.*/
  
                  } else {
  
                      updateConfirm(event);       /*Use the logic to confirm the event update properties*/
  
                      Evento.prototype.Update(event);     /*Modify the targeted event on the localStorage using the Update method of the Evento Class*/
  
                      $("#calendar").fullCalendar("updateEvent", event);
                      /*FullCalendar Method to report changes to an event and render them on the calendar.*/
  
                      $("#calendar").fullCalendar("refetchEvents");
                      /*FullCalendar Method to refetch events from all sources and rerender them on the screen.*/
  
                  }
              }
  </code></pre>
  
  <p>And here's a gif of the issue:
  <a href="https://i.imgur.com/rFPvvjE.gif" rel="noreferrer">https://i.imgur.com/rFPvvjE.gif</a></p>
  
  <p>UPDATE: With slicedtoad's help I isolated the issue to my updateConfirm logic:</p>
  
  <pre><code>var updateConfirm = function(event) {
      if (confirm("New title?")) {    /*Check if the user wants to change the event title*/
          event.title = prompt("Enter the new title: ");    /*Set new title for the event*/
      } else {
          event.title = event.title;
      }
  
      if (confirm("New description?")) {    /*Check if the user wants to change the event description*/
          event.description = prompt("Enter the new description: ");    /*Set new description for the event*/
      } else {
          event.description = event.description;
      }
  
      if (confirm("Is the event important?")) {   /*Check if the user wants to change the event priority*/
          event.overlap = false;
          event.backgroundColor = "red";    /*Set new priority for the event*/
      } else {
          event.overlap = true;
          event.backgroundColor = "blue";   /*Set new priority for the event*/
      }
  };
  </code></pre>
  
  <p>UPDATE 2:
  <code>console.log(event)</code> <strong>before</strong> updateConfirm(event):</p>
  
  <pre><code>Object {id: "2015-01-27T15:29:11+00:00", title: "título", start: m, end: m, allDay: false…}_allDay: false_end: m_id: "2015-01-27T15:29:11+00:00"_start: mallDay: falsebackgroundColor: "blue"className: Array[0]description: "gt4"end: mid: "2015-01-27T15:29:11+00:00"overlap: truesource: Objectstart: mstoringId: "2015-01-27T15:29:11+00:00"title: "título"__proto__: Object
  </code></pre>
  
  <p><code>console.log(event)</code> <strong>after</strong> updateConfirm(event):</p>
  
  <pre><code>Object {id: "2015-01-27T15:29:11+00:00", title: "título", start: m, end: m, allDay: false…}_allDay: false_end: m_id: "2015-01-27T15:29:11+00:00"_start: mallDay: falsebackgroundColor: "blue"className: Array[0]description: "gt4"end: mid: "2015-01-27T15:29:11+00:00"overlap: truesource: Objectstart: mstoringId: "2015-01-27T15:29:11+00:00"title: "título"__proto__: Object
  </code></pre>