def doit(a):
    b = []
    while len(a) > 0:
        x = a.pop(0)
        if not x in b or not x in a:
            b.append(x)
    return b


[G:/] $ python -i foo.py
>>> doit([1,2,3,3,2,1,1,2,3,2,2,3])
[1, 2, 3, 1, 2, 3]
>>> doit([0,1,0,3,0,0,0,0,0])
[0, 1, 3, 0]
>>>