The document contains a list of years, but some of them are repeated.
We need to write a script that leaves only unique elements by removing duplicates.
The order of the items in the list must be preserved.
Use the internal script.

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index.html

<!DOCTYPE html>
<html>
  <head>
    <title>Years</title>
  </head>
  <body>
    <h3>Years</h3>
    <ul>
      <li>1923</li>
      <li>1989</li>
      <li>1923</li>
      <li>1974</li>
      <li>1974</li>
      <li>1974</li>
      <li>1937</li>
      <li>1991</li>
      <li>1997</li>
      <li>1989</li>
    </ul>
  </body>
</html>