Pandas supports non-unique index values. If an operation that does not support duplicate index values is attempted, an exception will be raised at that time.
A basic difference between Series and ndarray is that operations between Series automatically align the data based on the label. Thus, we can write computations without considering whether all Series involved have the same label or not.
The result of an operation between unaligned Series (i.e. where the corresponding labels of the series are not the same or are not in the same order) will have the union of the indexes involved. If a label is not found in one Series or the other, the result will be marked as missing NaN. Being able to write code without doing any explicit data alignment grants immense freedom and flexibility in interactive data analysis and research.
Difference between Pandas Series and NumPy Arrays