title: Adding suffix to duplicate index values
body: <p>Here is a df:</p>
  
  <pre><code>-0.01   -0.029064
  -0.01   -0.032876
  -0.01   -0.040795
  -0.02   -0.027003
  -0.02   -0.0315
  </code></pre>
  
  <p>Need to concat this with another frame, but get the error "cannot reindex from a duplicate axis". What I'd like is a df like this:</p>
  
  <pre><code>-0.01   -0.029064
  -0.011   -0.032876
  -0.012   -0.040795
  -0.02   -0.027003
  -0.021   -0.031589
  </code></pre>
  
  <p>(note: added suffixes after each duplicate index)</p>