jquery is counting double the number of checkboxes..why is that?

I was wondering why Jquery counts double the number of checkboxes...here's my code:

$("#imprimir").click(function() {

  var selectedcb = [];
  var i = 0;

  $('input[type=checkbox]').each(function() {
    selectedcb.push($(this).attr('name'));
    alert(selectedcb[i]);
    i++;
  });
});

<script src="http://ift.tt/1oMJErh"></script>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb1" value="1" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb2" value="2" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb3" value="3" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb4" value="4" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb5" value="5" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb6" value="6" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb7" value="7" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb8" value="8" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb9" value="9" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb10" value="10" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb11" value="11" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb12" value="12" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb13" value="13" class="cb" checked>
  </center>
</th>
<th>
  <center>
    <input type="checkbox" id="cb" name="cb14" value="14" class="cb" checked>
  </center>
</th>

---

However it loops through the checkboxes twice, the first time it list all the checkboxes names only the checked checkboxes...can anyone explain this to me??