Remove duplicate without modifying a list while iterating through the same list (python)

Question

I have tried

def remove_extra(lst):
    return print(sorted(set(lst), key = lst.index))

def remove_extra(lst):
    b = []
    for x in lst:
        if x not in b:
            b.append(x)
    print(b)

All sort of removing duplicates codes however I don't know one that give me True when result1 is lst1 where result1 = remove_extra(lst1)

Answer 1

data = [1, 2, 3, 4, 4, 2, 1]

def remove_extra(lst):
    seen, result = set(), []
    for item in lst:
        if item not in seen:
            result.append(item)
            seen.add(item)
    lst[:] = result
    return lst

print data is remove_extra(data)
# True
print data
[1, 2, 3, 4]