Removing Duplicates from a Sequence

many solutions to this problem
1 hash (O(n))
  
from sets import Set as set
x = [1,3,5,1]
return list(set(x))
2 sort o(nlogn)
  
x.sort()
3 brute force (o(n^2))
result = []
for item in x:
   ....:     if item not in result:
   ....:         result.append(item)