title: How to duplicate Python dataframe one by one?
body: <p>I have a <code>pandas.DataFrame</code> as follows:</p>
  
  <pre><code>df1 = 
      a    b
  0   1    2
  1   3    4
  </code></pre>
  
  <p>I'd like to make this three times to become:</p>
  
  <pre><code>df2 =
      a    b
  0   1    2
  0   1    2
  0   1    2
  1   3    4
  1   3    4
  1   3    4
  </code></pre>
  
  <p><code>df2</code> is made from a loop, but it is not efficient.</p>
  
  <p>How can I get <code>df2</code> from <code>df1</code> using a matrix way which is faster?</p>