Bug: pct_change with frequency set as 'BM' throws value error

#### Code Sample, a copy-pastable example if possible
```
import pandas as pd
import random
import numpy as np


Creating the time-series index 
n=60
index = pd.date_range('01/13/2020', periods = 70,freq='D') 
  
Creating the dataframe  
df = pd.DataFrame({"A":np.random.uniform(low=0.5, high=13.3, size=(70,)), 
                   "B":np.random.uniform(low=10.5, high=45.3, size=(70,)),  
                   "C":np.random.uniform(low=70.5, high=85, size=(70,)), 
                   "D":np.random.uniform(low=50.5, high=65.7, size=(70,))}, index = index) 


df.pct_change(freq='BM')

```
#### Problem description

For a time-series data, the df.pct_change(freq='BM') doesn't works and throws following error.

ValueError: cannot reindex from a duplicate axis

#### Expected Output
df.asfreq('BM').pct_change()

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.1
numpy            : 1.15.4
pytz             : 2017.2
dateutil         : 2.6.1
pip              : 18.0
setuptools       : 39.1.0
Cython           : 0.26.1
pytest           : 5.1.2
hypothesis       : None
sphinx           : 1.6.3
blosc            : None
feather          : None
xlsxwriter       : 1.0.2
lxml.etree       : 4.1.0
html5lib         : 0.9999999
pymysql          : None
psycopg2         : 2.7.5 (dt dec pq3 ext lo64)
jinja2           : 2.9.6
IPython          : 6.1.0
pandas_datareader: None
bs4              : 4.6.0
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.1.0
matplotlib       : 2.1.0
numexpr          : 2.6.2
odfpy            : None
openpyxl         : 2.4.8
pandas_gbq       : None
pyarrow          : 0.8.0
pytables         : None
s3fs             : None
scipy            : 1.0.0
sqlalchemy       : 1.1.13
tables           : 3.4.2
xarray           : 0.10.7
xlrd             : 1.1.0
xlwt             : 1.3.0
xlsxwriter       : 1.0.2

</details>
