title: Flagging duplicate values in a column with pandas
body: <p>I have a df that looks like this:</p>
  
  <pre><code>email      is_new   col_n
  a@a        1           z 
  a@a        1           x      
  b@b        1           y 
  </code></pre>
  
  <p>I want to update the <code>is_new</code> column of the first instance of a email address. new df should look like this:</p>
  
  <pre><code>  email      is_new      col_n
      a@a        0           z 
      a@a        1           x      
      b@b        0           y 
  </code></pre>
  
  <p>I have tried creating <code>IF</code> statement to check the count of email address but not its not working:</p>
  
  <pre><code>   1.  if df[df["email"].groupby().unique()&gt; 1] ==True:
          print('ook')
  
     2. df.loc[df.groupby('email').groupby().unique(), 'is_new']=1
  </code></pre>