title: create Pandas Dataframe with unique index
body: <p>Can I create a dataframe which has a unique index or columns, similar to creating an unique key in mysql, that it will return an error if I try to add a duplicate index?</p>
  
  <p>Or is my only option to create an if-statement and check for the value in the dataframe before appending it?</p>
  
  <p>EDIT:</p>
  
  <p>It seems my question was a bit unclear. With unique columns I mean that we cannot have non-unique values in a column. </p>
  
  <p>With</p>
  
  <pre><code>df.append(new_row, verify_integrity=True)
  </code></pre>
  
  <p>we can check for all columns, but how can we check for only one or two columns?</p>