title: How to insert double letters
body: <p>So I am trying to figure some of these intricasies of python out just for shiggles now and I am trying to insert stuff into my strings.  I want to return a copy of the string all of the set letters doubled.  My letters are <code>l</code> <code>m</code> <code>n</code> <code>q</code> <code>t</code></p>
  
  <p>so my output for money would look like mmonney</p>
  
  <p>This is code from different attempt in which I stripped out letters, I am trying to figure how to change this so I double the letters in my list.  any suggestions?</p>
  
  <pre><code>def double(str):
    letters = ('l', 'm', 'n', 'q', 't')
    for char in str:
      if c.lower() in letters:
        str = str.replace(c,"")
      return str
  </code></pre>