Fix bug referencing the SAME set inside list.

Reproduce:

In [1]: list_of_sets = [set([0])] * 3

In [2]: list_of_sets
Out[2]: [{0}, {0}, {0}]

In [3]: list_of_sets[0].add(1)

In [4]: list_of_sets
Out[4]: [{0, 1}, {0, 1}, {0, 1}]
No double updating.