title: highcharts custom datalabels duplicating
body: <p>Can anyone tell me why the custom datalabels on my chart are displayed twice in some cases?  Note that this is not related to the bug in the export service, but I set textshadow to 'none' just in case. It does not seem to be consistent. Thanks in advance.  </p>
  
  <p>The JSFiddle is here:
  <a href="https://jsfiddle.net/zh3hvya3/" rel="nofollow">https://jsfiddle.net/zh3hvya3/</a></p>
  
  <pre><code>&lt;html&gt;
  &lt;script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs jquery/1.8.2/jquery.min.js"&gt;&lt;/script&gt;
  &lt;style type="text/css"&gt;
  #container {
      height: 400px; 
      min-width: 310px; 
      max-width: 800px;
      margin: 0 auto;
  }
  &lt;/style&gt;
  
  &lt;script type="text/javascript"&gt;          
  
  $(function () {
    window.chart1 = new Highcharts.Chart({
       chart: {
          renderTo: 'container',
          type: 'columnrange',
          inverted: true
       },
       title: {
          text: 'Tractor Utilization Chart'
       },
       xAxis: {
          title: {
              text: 'Vehicle'
          },
          categories: ['970106', '970108', '970110', '970111']
       },
       yAxis: {
          type: 'datetime',
          gridLineWidth: 1,
          gridLineColor: '#888888',
          min: 1463893200000,
          max: 1464498000000,
          tickInterval: 6 * 3600 * 1000,
          title: {
              text: 'Day and Time'
          }
       },
       legend: {
          enabled: true,
          labelFormatter: function() {
              return 'ABC';
          }
       },
       plotOptions: {
          columnrange: {
              grouping: false
          }
       },
       series: [{
          data: [{
              color: 'cyan',
              dataLabels: {
                  enabled: true,
                  align: 'left',
                  style: {
                      textShadow: 'none'
                  },
                  formatter: function() {
                      return '20614523';
                  }
              },
              x: 0,
              low: 1464057000000,
              high: 1464114900000
          }, {
              color: 'cyan',
              dataLabels: {
                  enabled: true,
                  align: 'left',
                  style: {
                      textShadow: 'none'
                  },
                  formatter: function() {
                      return '20614531';
                  }
              },
              x: 1,
              low: 1464060600000,
              high: 1464120660000
          }, {
              color: 'cyan',
              dataLabels: {
                  enabled: true,
                  align: 'left',
                  style: {
                      textShadow: 'none'
                  },
                  formatter: function() {
                      return '20614601';
                  }
              },
              x: 2,
              low: 1464048000000,
              high: 1464078538000
          }, {
              color: 'cyan',
              dataLabels: {
                  enabled: true,
                  align: 'left',
                  style: {
                      textShadow: 'none'
                  },
                  formatter: function() {
                      return '20614504';
                  }
              },
              x: 3,
              low: 1463967000000,
              high: 1464011852000
          }],
       }, {
          data: [{
              color: 'cyan',
              dataLabels: {
                  enabled: true,
                  align: 'left',
                  style: {
                      textShadow: 'none'
                  },
                  formatter: function() {
                      return '20614502';
                  }
              },
              x: 0,
              low: 1463947200000,
              high: 1463994392000
          }]
       }]
     });
  }); 
  
  &lt;/script&gt;
  &lt;/head&gt;
  
  &lt;body&gt;    
  &lt;script src="http://code.highcharts.com/highcharts.js"&gt;&lt;/script&gt;
  &lt;script src="http://code.highcharts.com/highcharts-more.js"&gt;&lt;/script&gt;
  &lt;script src="http://code.highcharts.com/modules/exporting.js"&gt;&lt;/script&gt;    
  &lt;div id="container" style="height: 400px"&gt;&lt;/div&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>