Create A Validation List

Hi all,
I have a list of letters in a column ie. A2 = A, A3 = B, A4 = C and so on.
What I want to do is add all of these letters to a drop down validation list in a cell.
I can do this fairly easily but only by adding each row as a different value (i.e. the user will have to chose between A, B or C).
What I want is for the user to be able to chose any combination of the letters (i.e. AB, AC, BC, etc).
I have found the following example on the internet:

Sub main()
Dim x As Long, sKills As String
For x = 2 To Sheets(1).Range("C65536").End(xlUp).Row
    sKills = sKills & Sheets(2).Cells(x, 3)
Next x

    ShowCombinations "", sKills
End Sub

Sub ShowCombinations(strPrefix As String, strMain As String)
    If strMain = "" Then
        Debug.Print strPrefix
        Exit Sub
    End If
    Dim strFirst As String, strRest As String
    strFirst = Left(strMain, 1)
    strRest = Mid(strMain, 2)
    ShowCombinations strPrefix & strFirst, strRest
    ShowCombinations strPrefix, strRest
End Sub

However, this does not return only the unique combinations (i.e. it returns ABC and CBA).

Can anyone advise me on how to do this?

Once I have all of the possible combinations, my plan is to use them to create a string and add that to the validation list in the cell.

I hope all of this makes sense?

Thanks snb (AGAIN!)
It always take me a while to decipher your code but it always turn out to be the most logical and efficent way of achieving what I need.