title: Adding a row to a Pandas DataFrame that would duplicate index
body: <p>I have a DataFrame with an index of type datetime objects. I am ultimately going to write this DataFrame to an HDF5 file using HDFStore.append. I am adding a lot of rows that need to be written to this HDF5 file. If i use HDFStore.append for every row, this takes way too long. If I collect everything in a DataFrame first, I run out of memory. So I need to chunk and write to HDF5 intermittently.</p>
  
  <pre><code>df = DataFrame([['Bob','Mary']], columns=['Boy', 'Girl'], index=[datetime.today()])
  </code></pre>
  
  <p>Now i would like to add another row to this WITH THE SAME INDEX</p>
  
  <pre><code>row = ['John', 'Sue']
  </code></pre>
  
  <p>Using .loc or .ix replaces the existing row</p>
  
  <pre><code>df.loc[datetime.today()] = row
  </code></pre>
  
  <p>Using append works, but for my purposes is WAY TOO SLOW</p>
  
  <pre><code>new_df = DataFrame([row], columns=df.columns, index=[datetime.today()])
  df.append(new_df)
  </code></pre>
  
  <p>Is there a better way to do this ?</p>