[Python] Strange behavior when partitioning list of tuples into different lists [duplicate]

I found some really unexpected behavior today using python 3.12.4. I am trying to sort a list of tuples into 2 different lists in a list of lists based on the first number of the tuple. Instead, each list in the list of lists gets every tuple.

I see similar posts here:

How to group a list of tuples/objects by similar index/attribute in python?

List of Tuples to List of List of Tuples

which suggests using defaultdict or itertools. But I would prefer to understand why what I am doing here does not work.

Here is a minimal recreation of the problem

data = [(0,1),(0,2),(1,1),(1,2),(1,3)]
arr = [[]]*2

for x in data:
if x[0] == 0:
arr[0].append(x)
elif x[0] == 1:
arr[1].append(x)
print(arr)
>>> [[(0, 1), (0, 2), (1, 1), (1, 2), (1, 3)], [(0, 1), (0, 2), (1, 1), (1, 2), (1, 3)]]


Hence, each list in the list of lists is the same and gets every tuple. My expectation is that the code should return:

>>> [[(0, 1), (0, 2)], [(1, 1), (1, 2), (1, 3)]]


Can anyone explain why this is happening?

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