title: How to check whether elements appears in the list only once in python?
body: <p>I have a list:</p>
  
  <pre><code>a = [1, 2, 6, 4, 3, 5, 7]
  </code></pre>
  
  <p>Please, explain to me how to check whether element appears only once in in the list?</p>
  
  <p>Please, also explain if all elements from 1 to <code>len(a)</code> are in the list. For instance, in list 'a' element from 1 to 7 are in the list, but if the list is <code>b = [1, 4, 3, 5]</code>, then not all elements from 1 to 4 are not in the list.</p>
  
  <p>Thank you!</p>