Write a function that takes a string and returns an array of the repeated characters (letters, numbers, whitespace) in the string.
If a charater is repeated more than once, only show it once in the result array.
Characters should be shown **by the order of their first repetition**. Note that this may be different from the order of first appearance of the character.
Characters are case sensitive.
For F# return a "char list"
## Examples:
```python
remember("apple") => returns ["p"]
remember("apPle") => returns []          # no repeats, "p" != "P"
remember("pippi") => returns ["p","i"]   # show "p" only once
remember('Pippi') => returns ["p","i"]   # "p" is repeated first
```
from collections import Counter

def remember(str_):
    seen, lst = Counter(), []
    for c in str_:
        if seen[c] == 1: lst.append(c)
        seen[c] += 1
    return lst
from collections import Counter

def remember(str_):
    seen, lst = Counter(), []
    for c in str_:
        if seen[c] == 1: lst.append(c)
        seen[c] += 1
    return lst