FindDupsRec.fs

let findDuplicateLetter list =
    let rec findDuplicate list item =
        match list with
        | [] -> "No duplicates were found"
        | x::xs when item = x -> x
        | x::xs -> findDuplicate xs x
    findDuplicate (List.sort list) ""

[<EntryPoint>]
let main argv = 
    printfn "%s" (findDuplicateLetter ["a";"b";"c"])
    printfn "%s" (findDuplicateLetter ["a";"a";"b";"c"])
    printfn "%s" (findDuplicateLetter ["a";"b";"b";"c"])
    printfn "%s" (findDuplicateLetter ["a";"b";"c";"c"])
    0 // return an integer exit code