title: How to duplicate a list in scheme?
body: <p>the following code removes duplicate in a list which I got from abo-abo's <a href="https://stackoverflow.com/questions/8382296/scheme-remove-duplicated-numbers-from-list">answer in this question</a>. </p>
  
  <pre><code>(define (remove-duplicates l)
    (cond ((null? l)
           '())
          ((member (car l) (cdr l))
           (remove-duplicates (cdr l)))
          (else
           (cons (car l) (remove-duplicates (cdr l))))))
  </code></pre>
  
  <p>I am just starting learning scheme, how can I duplicate each item in the list for example <code>(5 6 7 8 9 9 10 11 11)</code> will become <code>(5 5 6 6 7 7 8 8 9 9 9 9 10 10 11 11 11 11)</code>?</p>