Given a string, you have to return a string in which each character (case-sensitive) is repeated once.
```python
double_char("String") ==> "SSttrriinngg"
double_char("Hello World") ==> "HHeelllloo  WWoorrlldd"
double_char("1234!_ ") ==> "11223344!!__  "
```
Good Luck!
def double_char(s):
    l = list(s)
    r = []
    for i in range(len(l)):
        r.append(l[i]+l[i])
    return ''.join(r)
    
    

def double_char(s):
    l = list(s)
    r = []
    for i in range(len(l)):
        r.append(l[i]+l[i])
    return ''.join(r)
    
    
