title: Check for duplicate values in listbox and give error message
body: <p>I want to prevent from entering duplicate values in ListBox by giving an error message saying value has been duplicated in c#.</p>
  
  <p>E.g. </p>
  
  <pre><code>Listbox 
  Red|Blue|Red
  </code></pre>
  
  <p>Error: duplicate values has been entered.</p>