title: Trying to put same values in a item's list and getting ValidationException
body: <p>I recently decided to try out amazon-dynamodb and still trying to get a hold of it. </p>
  
  <p>In my use-case, i have to store two variables which have same values as a string list ("SS") in an item to DynamoDB. When i tried to do that, this is what i'm getting</p>
  
  <pre><code>[ValidationException: One or more parameter values were invalid: Input collection [X,X] contains duplicates.]
  message: 'One or more parameter values were invalid: Input collection [X, X] contains duplicates.',
  code: 'ValidationException',
  time: Fri Apr 25 2014 20:38:21 GMT+0000 (UTC),
  statusCode: 400
  </code></pre>
  
  <p>My question: Is there a way to store duplicate values in an item's list?</p>
  
  <p>Any knowledge regarding this will be appreciated. </p>