title: Pandas multiindex filter if duplicated, but keep first duplicated
body: <p>Assume I have a multiindex Pandas data frame with three indices (Date, Product, Country)</p>
  <pre><code>arrays = [['2021-12-30', '2021-12-30', '2021-12-30', '2021-12-31', '2021-12-31', '2021-12-31', '2022-01-01', '2022-01-01', '2022-01-02', '2022-01-02' ],
            ['A', 'A', 'B', 'A', 'A', 'B', 'B', 'B', 'B', 'B'], ['DE', 'IT', 'DE', 'DE', 'IT', 'DE', 'DE', 'IT', 'DE', 'IT']]
  
  index = pd.MultiIndex.from_arrays(arrays, names = ('Date', 'Product', 'Country'))
  df = pd.DataFrame(np.zeros(10), index = index)
  df
  
                               0
  Date       Product Country     
  2021-12-30 A       DE       0.0
                     IT       0.0
             B       DE       0.0
  2021-12-31 A       DE       0.0
                     IT       0.0
             B       DE       0.0
  2022-01-01 B       DE       0.0
                     IT       0.0
  2022-01-02 B       DE       0.0
                     IT       0.0
  </code></pre>
  <p>The data tells me which product (A and B) exists at what date in which country (DE and IT). I want to filter the data such that if Date &amp; Product is duplicated (meaning that a Product at a certain time exists in both countries), then the row with DE is removed. I found an intuitive way to do this:</p>
  <pre><code>df1 = df.index.to_frame()
  
  df = df[~df1.duplicated(subset=['Date','Product'], keep=False) | 
           df1['Country'].eq('IT')]
  
  df
  
                                0
  Date       Product Country     
  2021-12-30 A       IT       0.0
             B       DE       0.0
  2021-12-31 A       IT       0.0
             B       DE       0.0
  2022-01-01 B       IT       0.0
  2022-01-02 B       IT       0.0
  </code></pre>
  <p>I have however one further constraint that I am so far unable to consider. It tells me to keep both duplicated entries when it is duplicated for the first time (30.12. excluded). For example, on 30.12. and 31.12. product B is not duplicated, while it is the first time as of 01.01. Here '2022-01-01 B DE' should be kept, while it will be removed the day after.</p>
  <p>Does anybody have an idea how to do this? I would appreciate any help, many thanks in advance!</p>