Pandas Series duplicate index labels: When .loc returns a Series

The index in a Python Pandas series feels like a dict keys. But the keys can repeat:

s = pd.Series([10, 20, 30, 40], index=list('abca'))

s.loc['b']  # returns one value, np.int64(20)
s.loc['a']  # returns a series - both 'a' values, 10 and 40