def union(a,b):
    i = 0
    for e in b:
        if b[i] not in a:
            a = a.append(b[i])
        i += 1
    return a,b

a = [1,2,3]
b = [2,4,6]
union(a,b)
print a
#>>> [1,2,3,4,6]
print b
#>>> [2,4,6]