title: Mapbox-gl-draw duplicating points on drag when editing linestring from geojson
body: <p>When I select a point on the map and drag the point to move it, it duplicates the point and creates a new line between the two points surrounding the selected point, which ultimately results in a triangle instead of me being able to just modify the existing line.
  <a href="https://i.stack.imgur.com/ETMab.png" rel="nofollow noreferrer">triangle created by duplicated point</a></p>
  <p>I hastily threw this jsfiddle together but it demonstrates the issue i'm having when modifying an existing line (line is in blue, zoom in, click on it, select a point and draw):</p>
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>map.on('load', function () {
    map.addSource('multiple-lines-source', {
      'type': 'geojson',
      'data': geojson,
    });
  
    map.addLayer({
      'id': 'multiple-lines-layer',
      'type': 'line',
      'source': 'multiple-lines-source',
      'layout': {
      },
      'paint': {
        'line-color': 'black',
      },
    });
    
    let draw = new MapboxDraw()
    map.addControl(draw)
    draw.add(geojson)
  });</code></pre>
  </div>
  </div>
  </p>
  <p><a href="https://jsfiddle.net/badiguana/pgh6mxtL/19/" rel="nofollow noreferrer">jsfiddle</a></p>
  <p>Anyone know how to fix this behavior? I just want to be able to move points and reposition them without creating a new point. When I draw a line from scratch and modify a point on that line it works perfectly.</p>