Pandas merge or sum values in rows with the same index

When the values are numeric and apply operators:

df.groupby(level=0).sum()
df.groupby(df.index).sum()
df[specifiedColumn].groupby(level=0).sum()

When the values are numeric and apply functions:

# Common usage, merging values
df[1].groupby(level=0).apply(lambda xList: ';'.join(list([x for x in xList])))

Source:
https://pandas.pydata.org/pandas-docs/version/0.22/generated/pandas.core.groupby.GroupBy.apply.html
https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.GroupBy.sum.html