pandasIndexobjects support duplicate values. If a non-unique index is used as the group key in a groupby operation, all values for the same index value will be considered to be in one group and thus the output of aggregation functions will only contain unique index values: In [15]: lst ...
I know it is not a problem per se but I think there should be some pandas built in solution for this problem, as no simple function exists for this. It is largely discussed here with various ideas (https://stackoverflow.com/questions/12680754/split-pandas-dataframe-string-entry-to-separate...