title: append two data frame with pandas
body: <p>When I try to merge two dataframes by rows doing:</p>
  
  <pre><code>bigdata = data1.append(data2)
  </code></pre>
  
  <p>I get the following error:</p>
  
  <blockquote>
  <pre><code>Exception: Index cannot contain duplicate values!
  </code></pre>
  </blockquote>
  
  <p>The index of the first data frame starts from 0 to 38 and the second one from 0 to 48. I didn't understand that I have to modify the index of one of the data frame before merging, but I don't know how to.</p>
  
  <p>Thank you.</p>
  
  <p>These are the two dataframes:</p>
  
  <p><code>data1</code>:</p>
  
  <pre><code>    meta  particle  ratio   area    type    
  0   2     part10    1.348   0.8365  touching
  1   2     part18    1.558   0.8244  single  
  2   2     part2     1.893   0.894   single  
  3   2     part37    0.6695  1.005   single  
  ....clip...
  36  2     part23    1.051   0.8781  single  
  37  2     part3     80.54   0.9714  nuclei  
  38  2     part34    1.071   0.9337  single  
  </code></pre>
  
  <p><code>data2</code>:</p>
  
  <pre><code>    meta  particle  ratio    area    type    
  0   3     part10    0.4756   1.025   single  
  1   3     part18    0.04387  1.232   dusts   
  2   3     part2     1.132    0.8927  single  
  ...clip...
  46  3     part46    13.71    1.001   nuclei  
  47  3     part3     0.7439   0.9038  single  
  48  3     part34    0.4349   0.9956  single 
  </code></pre>
  
  <p>the first column is the index</p>