title: Double every letter in string
body: <p>What is the most efficient way in Python to double (or repeat n times) every letter in a string?</p>
  
  <pre><code>"abcd" -&gt; "aabbccdd"
  </code></pre>
  
  <p>or</p>
  
  <pre><code>"abcd" -&gt; "aaaabbbbccccdddd"
  </code></pre>
  
  <p>I have a long string that needs to be mutated in this fashion and the current solution involves a loop with <code>n</code> concatenations for every letter, which I imagine can be more efficient.</p>