title: problem with FullCalendar Events...events brought from the DB collide with the ones added with addEvent
body: <p>The title describes the problem.</p>
  <p>When the user makes a selection in FullCalendar, a dialogue appears with title, start, end data ...upon clicking add submit button the following happens:</p>
  <ul>
  <li>ajax call to the db to add the event</li>
  <li>and call to calendar.addEvent</li>
  </ul>
  <p>Here is some code to demo the above:</p>
  <pre><code> if($('#new_appt_data').valid())
   {
           $.ajax({
                type: &quot;POST&quot;,
                url: &quot;events.php&quot;,
                dataType:&quot;json&quot;,
                data:{&quot;insert&quot;:true,&quot;title&quot;:title,&quot;start&quot;:fromDtime,&quot;end&quot;:toDtime,&quot;services&quot;:servicestatus,&quot;staff&quot;:staffstatus,&quot;origin&quot;:'backend',&quot;packagetype&quot;:pack},
                success:function(data){
                 var eventData = {id:data,title: title,origin:'backend', start:fromcal, end:endcal,services: servicestatus,staff:staffstatus};
                              calendar.addEvent(eventData);
                              $(&quot;#new_appt&quot; ).dialog( &quot;close&quot; );
                            
                     },
                error:function(jqXHR, textStatus, errorThrown)
                            { 
                                $('#err_msg_new_appt').text('problem') 
                            } 
  
                  });
  </code></pre>
  <p>Above you can see also non-standard event object fields...such as origin.</p>
  <p>On the success callback I use addEvent to add the event to the Cal.</p>
  <p>And now the problem:
  This is an image when the event is added to the Cal..the week is 19-25 Apr.(sorry for the Greek)
  <a href="https://i.stack.imgur.com/yGrIZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yGrIZ.png" alt="when event added" /></a></p>
  <p>Imagine now going  1 week ahead and the returning to the initial week(that the event was added...namely 19-25Apr) ... here is the image:
  <a href="https://i.stack.imgur.com/cs5LM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cs5LM.png" alt="enter image description here" /></a></p>
  <p>You see the same event appearing twice. It seems that the one event was the event added with addEvent FC method and the other comes as JSON from the database.</p>
  <p>Why does this happen? Is there something wrong with my code (mainly from a logic point of view), or is it a FC bug?</p>
  <p>I must also add that refreshing solves the problem...</p>