Operations between series require non-duplicated indices, otherwise Pandas doesn’t know how to align values in calculations. This isn’t the case with your data currently.
If you are certain that your series are aligned by position, you can call reset_index on each dataframe:
wind = pd.DataFrame({'DATE (MM/DD/YYYY)': ['2018-01-01', '2018-02-01', '2018-03-01']})
temp = pd.DataFrame({'stamp': ['1', '2', '3']}, index=[0, 1, 1])
# ATTEMPT 1: FAIL
wind['timestamp'] = wind['DATE (MM/DD/YYYY)'] + ' ' + temp['stamp']
# ValueError: cannot reindex from a duplicate axis
# ATTEMPT 2: SUCCESS
wind = wind.reset_index(drop=True)
temp = temp.reset_index(drop=True)
wind['timestamp'] = wind['DATE (MM/DD/YYYY)'] + ' ' + temp['stamp']
print(wind)
DATE (MM/DD/YYYY) timestamp
0 2018-01-01 2018-01-01 1
1 2018-02-01 2018-02-01 2
2 2018-03-01 2018-03-01 3
