replaceDuplicates.fs

 module Array =
        open System.Collections.Generic

        let toMap<'TItem when 'TItem : equality> array = 

            let map = array |> Seq.fold (fun (map: Dictionary<'TItem, int>) ch ->
                        if map.ContainsKey(ch) then 
                            map.[ch] <- map.[ch] + 1
                        else
                            map.Add(ch, 1)
                        map) (new Dictionary<'TItem, int>())
            map
            
let replaceExact (source: string) (find: string) (replace: string) = 
        let pattern = String.Format(@"\b{0}\b", find);
        Regex.Replace(source, pattern, replace)

let replaceDuplicates str =
    match str with
         | null | "" -> str
         | _ ->
             let inline replaceWithWhitespaces acc key = replaceExact acc key (String.replicate (String.length key) " ");

             str.Split(" ", StringSplitOptions.None) 
                |> Array.toMap 
                |> fun map -> map.Keys |> Array.ofSeq |> Array.filter (fun key -> map.[key] > 1)
                |> fun duplicates -> Array.fold replaceWithWhitespaces str duplicates