s = 'arefhjaref!!'
out = []
for i in range(len(s)):
    t = s[i:]
    for j in range(2,len(t)):
        s1,s2 = t[:j],t[j:]
        if s2.find(s1) < 0: 
            continue
        out.append(s1)
out.sort(key = lambda x: len(x),reverse=True)
print(out)

 s = 'arefhjjjjaref!!'
out = []
for i in range(len(s)):
    t = s[i:]
    for j in range(2,len(t)):
        s1,s2 = t[:j],t[j:]
        if s2.find(s1) < 0:
            continue
        out.append(s1)
out.sort(key = lambda x: len(x),reverse=True)
print(out)

>>>['aref', 'are', 'ref', 'ar', 're', 'ef', 'jj']

   
from itertools import takewhile
import random
import timeit
def find_longest_substr1(string):
    longest_seq = ""
    s = string
    while s:
        char = s[0]
        seq = list(takewhile(lambda i: i == char, s))
        length = len(seq)
        if length > len(longest_seq):
            longest_seq = seq
        s = s[length:]
    return "".join(longest_seq)
def find_longest_substr2(string):
    out = []
    for i in range(len(string)):
        t = string[i:]
        for j in range(2, len(t)):
            s1,s2 = t[:j], t[j:]
            if s2.find(s1) < 0:
                continue
            out.append(s1)
    out.sort(key=lambda x: len(x), reverse=True)
    return out
_s = ["a", "b", "c", "d", "e"]
s = "".join(random.choice(_s) for _ in range(1000))
print(timeit.timeit("find_longest_substr1(s)", setup="from __main__ import find_longest_substr1, s", number=1))
print(timeit.timeit("find_longest_substr2(s)", setup="from __main__ import find_longest_substr2, s", number=1))

>>>0.0018036999999999984
>>>0.5519485999999999

 import timeit
def test():
    from itertools import takewhile
print(timeit.timeit("test", setup="from __main__ import test", number=1))

 3.9211334579292156e-07