Why does it work? I thought that a set doesn't keep the order of the elements.

for i in range(0, len(string), k): print(''.join(list(set(string[i : i + k]))))

another one liner solution (python 3.7+)

    print( *[ "".join( list( dict.fromkeys(string[i: i+k]) ) ) for i in range(0, len(string), k)], sep="\n")

l = len(string) // k print(l)

for i in range(0, len(string), k):
    s = ''
    s += string[i]
    g=0

    for j in range(i + 1, i + k):
        c = False
        g += 1
        for h in range(g):

            if string[j:j + 1] != string[i + h]:
                c = True

            else:
                c = False
                break
        if c:
            s += string[j:j + 1]
    print(s)