title: How do I add unique elements to a List?
body: <p>In flutter/dart, how can I add elements to a list whilst ensuring that every new entry is unique (i.e. no duplicates). </p>
  
  <p>So we know how to add elements to a list:</p>
  
  <pre><code>List myList = [];
  </code></pre>
  
  <p>then</p>
  
  <pre><code>myList.add(...);
  </code></pre>
  
  <p>but I'd like to know how to make sure that whatever has been added has not been added before.</p>
  
  <p>Many thanks,</p>