Random numbers without duplicates

Simulating a lottery of 6 numbers chosen from 40, I want to create a list of numbers in Haskell using the system random generator but eliminate duplicates, which often arise.

If I have the following:

import System.Random

main :: IO ()
main = do
  rs <- forM [1..6] $ \_x -> randomRIO (1, 40) :: (IO Int)
  print rs

this is halfway. But how do I filter out duplicates? It seems to me I need a while loop of some sort to construct a list filtering elements that are already in the list until the list is the required size. If I can generate an infinite list of random numbers and filter it inside the IO monad I am sure that would work, but I do not know how to approach this. It seems while loops are generally deprecated in Haskell, so I am uncertain of the true Haskeller's way here. Is this a legitimate use case for a while loop, and if so, how does one do that?