BUG: IntervalIndex.intersection returning duplicates

[phofl]

• closes BUG: IntervalIndex intersections are not intersections in many cases #38743
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Moved the wrong test code into a new test with adjusted behavior

[jreback]

can you also add the exhaustive test from the op

def check_intersection_commutative(left, right):
    assert left.intersection(right).equals(right.intersection(left))

@pytest.mark.parametrize("index_maker", tm.index_subclass_makers_generator())
def test_intersection_uniqueness(index_maker):
    if index_make is tm.makeMultiIndex:
        pytest.pass()  # This index maker is bugged
    ind_unique = index_maker(k=5)
    ind_non_unique = idx_unique[[0, 0, 1, 2, 3, 4]]

    check_intersection_commutative(ind_unique, ind_non_unique)
    assert idx_unique.intersection(idx_non_unique).is_unique

except use the index fixture

[phofl]

Done, moved it into the regular setops file.

[jreback]

thanks @phofl

[jbrockmendel]

ive been thinking we should make something like this of the form

@pytest.mark.parametrize("method", ["union", "intersection", "symmetric_difference"])
def test_setops_commute(method, index, index_fixture2):
    [...]

we've consolidated a lot of the behavior recently, but we still have some length-0 checks and self.equals(other) checks that seem a little wobbly

[phofl]

Yeah, that sounds reasonable