Pandas error: cannot reindex from a duplicate axis

I have a dateframe named Mj_rank, with date as Datetime and index which looks like this:

                A      B     C ...
date
2016-01-29     False  False  True
2016-01-30     False  False  True
2016-02-01     True   True   True
  ....
2017-12-29     False  True   True

Currently, the data is daily, but I would like to resample the data into a new df that contains every 6 months nth.

Therefore I did:

Mj_rank_s = Mj_rank.resample('6M').asfreq().tail()

which gives me this output:

ValueError: cannot reindex from a duplicate axis

strangely enough, if I use other methods like max() or min() it works fine, but not "asfreq()".

I tried different ways based on existing stackoverflow suggestions like adding in front, but didn't work :

Mj_rank = Mj_rank.reset_index()
Mj_rank['date'] = pd.to_datetime(Mj_rank['date'])
Mj_rank = Mj_rank.set_index('date')

Thanks a lot!

Edit: Thanks to @jezrael he pointed out I had problems with duplicates using Mj_rank[Mj_rank.index.duplicated(keep=False)]