Elm - get only one element of value from list

main.elm

unique : List a -> List a
unique list =
    case list of
        [] ->
            []

        x :: [] ->
            x :: []

        x :: xs ->
            if List.member x xs then
                unique xs
            else
                x :: unique xs
                
-- unique [1,2,3,3,3,3] -> [1,2,3]

You can also do it shorter

getUnique : List comparable -> List comparable 
getUnique =
  Set.toList << Set.fromList