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])))

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