Why is one duplicate not removed?

[Yoriz]

You shouldn't alter a list while iterating over it, it loses track of the position, make a copy of the list to iterate and alter the original.

def removeDups(L1, L2):
    """Assumes that L1 and L2 are lists.
    Removes any element from L1 that also occurs in L2"""
    for e in L1[:]:
        if e in L2:
            L1.remove(e)


L1 = [1, 2, 3, 4, 6]
L2 = [1, 2, 5, 6]
removeDups(L1, L2)
print('L1 =', L1)

Output:
L1 = [3, 4]