First Character that repeats

DESCRIPTION:

Find the first character that repeats in a String and return that character.

first_dup('tweet') => 't'
first_dup('like') => None

This is not the same as finding the character that repeats first. In that case, an input of 'tweet' would yield 'e'.

Another example:

In 'translator' you should return 't', not 'a'.

v      v  
translator
  ^   ^

While second 'a' appears before second 't', the first 't' is before the first 'a'.

---

def first_dup(s):
    for i in range(len(s)):
        if s[i] in s[i+1:]:
            return s[i]