In this kata you have to create all permutations of an input string and remove duplicates, if present. This means, you have to shuffle all letters from the input in all possible orders.
Examples:
```python
permutations('a'); # ['a']
permutations('ab'); # ['ab', 'ba']
permutations('aabb'); # ['aabb', 'abab', 'abba', 'baab', 'baba', 'bbaa']
```
The order of the permutations doesn't matter.
def permutations(string):
  if len(string) == 1: return set(string)
  first = string[0]
  rest = permutations(string[1:])
  result = set()
  for i in range(0, len(string)):
    for p in rest:
      result.add(p[0:i] + first + p[i:])
  return result
def permutations(string):
  if len(string) == 1: return set(string)
  first = string[0]
  rest = permutations(string[1:])
  result = set()
  for i in range(0, len(string)):
    for p in rest:
      result.add(p[0:i] + first + p[i:])
  return result