Find the first character that repeats in a String and return that character. 
```python
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'.*
def first_dup(stg):
    return next((c for c in stg if stg.count(c) > 1), None)
def first_dup(stg):
    return next((c for c in stg if stg.count(c) > 1), None)