def double_substring(line):
    """
        length of the longest substring that non-overlapping repeats more than once.
    """
    result = []
    for i in range(len(line) - 1):
        for j in range(i + 1, len(line) + 1):
            if line[i:j] in line[j:]:
                result.append(len(line[i:j]))

    if len(result) > 0:
        return max(result)
    else:
        return 0


if __name__ == '__main__':
    # These "asserts" using only for self-checking and not necessary for auto-testing
    assert double_substring('aaaa') == 2, "First"
    assert double_substring('abc') == 0, "Second"
    assert double_substring('aghtfghkofgh') == 3, "Third"