title: How to save Item in dynamodb with GSI condition?
body: <p>I have a dynamodb table that has a Global secondary Index with a range key (email, hashedPassword ).
  i want to save an item if the email is not duplicated,
  i used <code>attribute_not_exists</code> but it doesn't work, i also used :</p>
  
  <pre><code>ConditionExpression: "#email &lt;&gt; :email",
  ExpressionAttributeNames: {"#email": "email"},
  ExpressionAttributeValues: {":email": userInfo.email}
  </code></pre>
  
  <p>without success.</p>
  
  <p>Can anyone help me please,</p>
  
  <p>Thank you.</p>