编写一个Python函数，找出一个字符串中所有重复的字符。


字符串: "hello world"
```python
def find_duplicates(s):
    dup = []
    for c in s:
        if s.count(c) > 1 and c not in dup:
            dup.append(c)
    return dup

s = "hello world"
print(find_duplicates(s))
```

输出:
```python
['l', 'o']
```