title: Mongo find duplicates for entries for two or more fields
body: <p>I have documents like this:</p>
  
  <pre><code>{
      "_id" : ObjectId("557eaf444ba222d545c3dffc"),
      "foreing" : ObjectId("538726124ba2222c0c0248ae"),
      "value" : "test",
  }
  </code></pre>
  
  <p>I want to find all documents which have duplicated values for pair <code>foreing</code> &amp; <code>value</code>.</p>