Mongo find duplicates for entries for two or more fields
<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>