title: Adding duplicate rows to a DataFrame
body: <p>I did not figure out how to solve the following question!
  consider the following data set:</p>
  
  <pre><code>df = pd.DataFrame(data=np.array([['a',1, 2, 3], ['a',4, 5, 6],
                                   ['b',7, 8, 9], ['b',10, 11 , 12]]),
   columns=['id','A', 'B', 'C'])
  
    id   A    B    C
    a    1    2    3
    a    4    5    6
    b    7    8    9
    b    10   11   12
  </code></pre>
  
  <p>I need to group the data by id and in each group duplicate the first row and add it to the dataset like the following data set:</p>
  
  <pre><code>  id   A    B    C    A  B  C
    a    1    2    3    1  2  3
    a    4    5    6    1  2  3
    b    7    8    9    7  8  9
    b    10   11   12   7  8  9
  </code></pre>
  
  <p>I really appreciate it for your help.</p>
  
  <p>I did the following steps, however I could not expand it :</p>
  
  <pre><code>df1 = df.loc [0:0 , 'A' :'C']
  df3 = pd.concat([df,df1],axis=1)
  </code></pre>