title: cannot add an entity with a key that is already in use
body: <p>I have got this weird error <code>Cannot add an entity with a key that is already in use</code>
  But what is quite irritable about that error is that user gets no detais - Who? What? What table? What record is the culprit of this error?</p>
  
  <p>It would be desperately complicated to determine it, in case you do many operations on LINQ objects before <code>.Submit()</code></p>
  
  <p>Is there any way to determine what certainly caused this error?</p>