title: DBConcurrencyException when trying to return NotFound() in Update
body: <p>I try to implement Microsofts case for an API Update which is shown below.
  First I get the record from the database to determin if it is null. If not, an update will be triggered.
  In by code efCore throws an DBConcurrencyException because the element has alread been tracked.</p>
  <p>Whats the problem?</p>
  <pre><code>// PUT api/contactsconvention/{guid}
  [HttpPut(&quot;{id}&quot;)]
  [ApiConventionMethod(typeof(DefaultApiConventions), 
                       nameof(DefaultApiConventions.Put))]
  public IActionResult Update(string id, Contact contact)
  {
      // First tracking in my code
      var contactToUpdate = _contacts.Get(id);
  
      if (contactToUpdate == null)
      {
          return NotFound();
      }
  
      // Second tracking in my code -&gt; Exception at dbContext.SaveChanges()
      _contacts.Update(contact);
  
      return NoContent();
  }
  </code></pre>