title: SQL query returns double data
body: <pre><code> var result = Context.ItemSet
      .OrderBy(o =&gt; o.Index)
      /*.Include(t =&gt; t.Group) with this, item with id 500 is outputted 4 times, without it is outputted 2 times*/
      .ToList();
  
   for (int i = 0; i &lt; result.Count(); i++)
   {
        Debug.WriteLine("-"+i);
        Debug.WriteLine(result[i].Id);
   }
  </code></pre>
  
  <p>The above code outputs the following:</p>
  
  <pre><code>&gt;      -0
  &gt;       1
  &gt;      -1
  &gt;      11
  &gt;      -2
  &gt;      12
  &gt;      -3
        ...
  &gt;     -30
  &gt;      17
  &gt;     -31
  &gt;     206
  &gt;     -32
  &gt;     500 //
  &gt;     -33
  &gt;     500 //duplicate
  &gt;     -34
  &gt;     203
  </code></pre>
  
  <p>Why does the resultlist contain the same data multiple times and how can I prevent that? I can<em>not</em> use <code>select distinct id</code> because I need <strong>all</strong> columns.</p>