title: Sequence contains more than one matching element
body: <p>When I am trying to set <code>IsDefault</code> property of each dressing items where match a condition it throws an error saying:</p>
  
  <blockquote>
    <p>Sequence contains more than one matching sequence.</p>
  </blockquote>
  
  <pre><code>(this.DressingItems
       .Where(xx =&gt; xx.DressingInfo.CatID == catId 
                          &amp;&amp; xx.ProductID == this.ProductID)
       .Single()).IsDefault = false;
  </code></pre>