title: Python 3 - Check letters in string with the next letter
body: <p>If I have a string </p>
  
  <pre><code>String = 'ABCEEFGH'
  </code></pre>
  
  <p>How can I check what letter is beside each letter without going out of index?</p>
  
  <pre><code>for index in range(len(String)):
      if String[index] == String[index+1]: 
          print('Double')
  </code></pre>