title: Finding duplicate entries in Collection
body: <p>Is there a tool or library to find duplicate entries in a Collection according to specific criteria that can be implemented?</p>
  
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  <p>To make myself clear: I want to compare the entries to each other according to specific criteria. So I think a <code>Predicate</code> returning just <code>true</code> or <code>false</code> isn't enough.</p>
  
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  <p>I can't use <code>equals</code>.</p>