But if duplicated values in index get error:

df = pd.concat([df, df])
df = df.set_index('Date')

print (df.asfreq('d').index)

ValueError: cannot reindex from a duplicate axis

Solution is use resample with some aggregate function:

print (df.resample('2D').mean().index)

DatetimeIndex(['1973-05-01', '1973-05-03', '1973-05-05', '1973-05-07',
               '1973-05-09', '1973-05-11', '1973-05-13', '1973-05-15',
               '1973-05-17', '1973-05-19', '1973-05-21', '1973-05-23',
               '1973-05-25', '1973-05-27', '1973-05-29', '1973-05-31',
               '1973-06-02', '1973-06-04', '1973-06-06', '1973-06-08',
               '1973-06-10', '1973-06-12', '1973-06-14', '1973-06-16',
               '1973-06-18', '1973-06-20', '1973-06-22', '1973-06-24',
               '1973-06-26', '1973-06-28', '1973-06-30', '1973-07-02',
               '1973-07-04', '1973-07-06', '1973-07-08', '1973-07-10',
               '1973-07-12', '1973-07-14', '1973-07-16', '1973-07-18',
               '1973-07-20', '1973-07-22', '1973-07-24', '1973-07-26',
               '1973-07-28', '1973-07-30', '1973-08-01', '1973-08-03',
               '1973-08-05', '1973-08-07', '1973-08-09', '1973-08-11',
               '1973-08-13', '1973-08-15', '1973-08-17', '1973-08-19',
               '1973-08-21', '1973-08-23', '1973-08-25', '1973-08-27',
               '1973-08-29', '1973-08-31', '1973-09-02', '1973-09-04',
               '1973-09-06', '1973-09-08', '1973-09-10', '1973-09-12',
               '1973-09-14', '1973-09-16', '1973-09-18', '1973-09-20',
               '1973-09-22', '1973-09-24', '1973-09-26', '1973-09-28',
               '1973-09-30'],
              dtype='datetime64[ns]', name='Date', freq='2D')