How can I remove duplicates in an F# sequence without using references

Question

I have a sorted sequence and want to go through it and return the unique entries in the sequence. I can do it using the following function, but it uses reference variables a

Answer 1

The solution below, preserves order of elements and returns only the first occurance of an element in a generic list. Of course this generates a new List with the redundant items removed.

//  ****  Returns a list having subsequent redundant elements removed
let removeDuplicates(lst : 'a list) = 
    let f item acc =
        match acc with 
        | [] -> [item]
        | _ ->
            match List.exists(fun x -> x = item) acc with
            | false -> item :: acc
            | true -> acc
    lst 
    |> List.rev
    |> fun x -> List.foldBack f x []
    |> List.rev
//  **** END OF FUNCTION removeDuplicates

val removeDuplicates : 'a list -> 'a list when 'a : equality
val testList : int list = [1; 4; 3; 1; 2; 2; 1; 1; 3; 4; 3]
val tryAbove : int list = [1; 4; 3; 2]