Write a python program to find Largest Substring that occurs more than once

def get_longest_substring(str):
    n = len(str)
    l = [[0 for x in range(n + 1)]
            for y in range(n + 1)]
    res = ""
    res_length = 0
    idx = 0
    for i in range(1, n + 1):
        for j in range(i + 1, n + 1):
            if (str[i - 1] == str[j - 1] and
                l[i - 1][j - 1] < (j - i)):
                l[i][j] = l[i - 1][j - 1] + 1
                if (l[i][j] > res_length):
                    res_length = l[i][j]
                    idx = max(i, idx)

            else:
                l[i][j] = 0
    if (res_length > 0):
        for i in range(idx - res_length + 1,idx + 1):
            res = res + str[i - 1]

    return res

print(get_longest_substring("aadaghgdjdjdjaada"))