title: How to find specific Elements in Lists which are part of Lists
body: <p>My problem is::</p>
  
  <p>I have a List of instances of a class "company"
  each company contains a list of a class "Car" and an unique id
  each car has a lot of specific data like "brand" an "model"</p>
  
  <p>My task is to create a new list which contains all duplicate cars with the companys id who use them. In each "car" class is an object <code>licensplateNO</code> which is also unique. So to check only for duplicate instances of Cars is no option because each instance is different in the licenseplate property. My last Idea would be to do it the old fashioned way with a lot of loops comparing each property of each element with each other.</p>
  
  <p>the result should look like this:</p>
  
  <pre><code>list duplicateCars: 
  Car1: BMW M4     used by Company B, Company F, Company X
  Car2: Audi A3    used by Company B, Company D
  Car3: VW  Golf   used by Company D, Company F .....
  </code></pre>