title: Dart - find duplicate values on a List
body: <p>how can I find duplicate values on a list,
  Let's say I got a List like this:</p>
  
  <pre><code>List&lt;Map&lt;String, dynamic&gt;&gt; users = [
      { "name": 'John', 'age': 18 },
      { "name": 'Jane', 'age': 21 },
      { "name": 'Mary', 'age': 23 },
      { "name": 'Mary', 'age': 27 },
    ];
  </code></pre>
  
  <p>How I can iterate the list to know if there are users with the same name?</p>