# dominant_substring.py

# dominant_substring.py

def dominant_substring(string):
    curr, subs = '', ''
    for char in string:
        if not curr or string.count(curr+char) > 1:
            curr += char
        else:
            if string.count(curr)*len(curr) > string.count(subs)*len(subs):
                if len(curr) > 1:
                    subs = curr
            curr = ''
    return subs