title: JSON duplicated properties validation via Jackson
body: <p>I use Jackson and want to check that input JSON string doesn't contain duplicated properties like:</p>
  
  <p><code>{"a" : 1, "a" : 0}</code></p>
  
  <p>Following Jackson fragment process input string without any errors and even return value:</p>
  
  <p><code>
   JsonNode jsonSelect = mapper.readTree("{ A : 1, A : 0}");
   System.out.println(jsonSelect.getFieldValue("A")); // prints 0
  </code></p>
  
  <p>Does I have a chance to validate duplicates via Jackson?</p>
  
  <p>P.S. Does JSON format support duplicated properties at all? I didn't find any restrictions about it in <a href="http://www.ietf.org/rfc/rfc4627.txt" rel="nofollow">specification</a>. Also <code>org.json.JSONObject</code> throws an exception for duplicates that doesn't give me an answer - is <code>{"a" : 1, "a" : 0}</code> well-formed according to standard.</p>