How do I tag every author in their posts that they have made previously

I would do this by looking into the wp_get_post_revisions

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So put this in your functions.php:

function show_revisions( $revisions ){
  echo '<ul>';
  foreach( $revisions as $revision ):

    // Gets the author if there is one.
    // Prints (??) if there isn't one.
    $author = empty( ucfirst( get_the_author_meta( 'display_name', $post_author ) ) ) ? '(??)' : ucfirst( get_the_author_meta( 'display_name', $revision->post_author ) );

      echo '<li>';
      echo '<p>Author:' . $author . '</p>';
      echo '</li>';
  endforeach;
  echo '</ul>';
}

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And put this in your single.php and/or page.php and/or whereever you want the list of authors to be shown:

$revisions = wp_get_post_revisions();
show_revisions( $revisions );