title: If duplicate then str.replace in Pandas
body: <p>I'm trying to create a way to search through a column and if it's a duplicate value, replace a string. This is what I have so far</p>
  <pre><code>date
  November 1st 2020
  November 2nd 2020
  November 3rd 2020
  November 1st 2020
  November 2nd 2020
  November 3rd 2020
  November 1st 2020
  November 2nd 2020
  November 3rd 2020
  </code></pre>
  <p>And what I'm trying to do is this</p>
  <pre><code>date
  November 1st 2020 first instance
  November 2nd 2020 first instance
  November 3rd 2020 first instance
  November 1st 2020 second instance
  November 2nd 2020 second instance
  November 3rd 2020 second instance
  November 1st 2020 third instance
  November 2nd 2020 third instance
  November 3rd 2020 third instance
  </code></pre>
  <p>Is there a way to do this?
  <a href="https://stackoverflow.com/questions/39907315/replace-duplicate-values-across-columns-in-pandas">Replace duplicate values across columns in Pandas</a>
  This will only add &quot;1 character&quot; to a dataframe, so would you create a loop in order to keep adding 1 or what ever you would want to it? Something like</p>
  <pre><code>pd.Series(['date']).duplicated()
  is_duplicate = df.apply(pd.Series.duplicated, axis=1)
  for is_duplicate
     df.where(~is_duplicate, +1)
  </code></pre>
  <p>I don't really understand how to iterate through the columns to get the desired result.</p>