title: Throw exception when a property is duplicated in a properties file
body: <p>How can I throw an Exception when a properties file contains a duplicate property?
  Here is an example demonstrating this situation:</p>
  
  <pre><code># Properties-file
  
  directory=D:\\media\\D-Downloads\\Errorfile\\TEST_A
  directory=D:\\media\\D-Downloads\\Errorfile\\TEST_B
  #directory=D:\\media\\D-Downloads\\Errorfile\\TEST_C
  </code></pre>