限制用户在自动完成中选择相同的值

Question

我使用ajax自动完成功能选择多个值。

选择了多个值，但问题是它还接受之前选择的相同值。

我的脚本功能是

<script>
$(function() {
  function split( val ) {
    return val.split( /,\s*/ );
  }
  function extractLast( term ) {
    return split( term ).pop();
  }
  var url = "<?php echo $this->createUrl('tasks/autocompletedeveloper') ?>";
  $( "#developers" ).bind( "keydown", function( event ) {
    if ( event.keyCode === $.ui.keyCode.TAB &&
        $( this ).autocomplete( "instance" ).menu.active ) {
        event.preventDefault();
    }
  })

  .autocomplete({
    minLength: 1,
    source: function( request, response ) {
        // delegate back to autocomplete, but extract the last term
        $.getJSON(url, { term : extractLast( request.term )},response);
    },
    focus: function() {
        // prevent value inserted on focus
        return false;
    },
    select: function( event, ui ) {
        var terms = split( this.value );
        terms.pop();
        terms.push( ui.item.value );
        terms.push( "" );
        this.value = terms.join( ", " );
        return false;
    }
  });
});
</script>

我的HTML代码是

<div class="form-group">    
    <label>Developer</label>
    <input id="developers" class="form-control" /> 
</div>

请帮我限制相同的值选择。