Using a recordset to display unique values within a field in summary

I'm using a function to concatenate values (separated by a comma) of a field in a summarized data set. I have a data set that lists the types of items (eg. B, P, F, D) associated with an order.


Order_ID     Prod_Type
A11            B
A11            P
A11            P
A11            D

Currently, the function I am using will return "B, P, P, D" for A11. I want it however to identify the unique values and return "B,P,D" instead. How can I modify my code to do this?

Function Route(ORD_Str) As String
Dim db As DAO.Database
Dim strSQL As String
Dim rst As DAO.Recordset
Dim PTypes As String
 
strSQL = "SELECT Prod_Type FROM Order_Detail WHERE Ord_Id='" & 	ORD_Str & "' ORDER BY req_dt desc"
Set db = CurrentDb
Set rst = db.OpenRecordset(strSQL, dbOpenSnapshot)
 
Do Until rst.EOF
    PTypes = PTypes & ", " & rst!Prod_Type
    rst.MoveNext
Loop
 
Route = Right(PTypes, Len(PTypes) - 2)
 
End Function

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