MS Access: dcount duplicate rows?

我的代码是可以运作，但是有一些问题，当我创建一个新的纪录，这无法创建记录到 OrderID2，由于订单Order1已经存在的记录。
我想创建一个基于订单ID记录，但产品ID不能重复
该表将是：
Order ID      Product ID
1                  2
1                  3
1                  4
2                  2
2                  3

2                  4

Private Sub AddToOrder_Click()


On Error GoTo Err_AddToOrder_Click


 
If DCount("[OrderID]", "T_Order_line", "ProductID=" & Me.ProductID) Then
        'Counts and compares the records in "T_Order_line" and ensures that records are not duplicated.
    Me.ProductID.Undo
        'cancels an entry if a duplicate is found
    MsgBox "Warning! ProductID." _
                 & Me.ProductID & vbCr & "has already been entered into this order " _
                 & vbCr & vbCr & "Please check ordered items", _
                 vbInformation, "Duplicate Information"
    MsgBox "Please select another product to add to current order", _
               vbInformation, "Duplicate Information"


Else



    Forms![F_Orders]![F_Order_line_subform].SetFocus
    Me.ProductID.SetFocus
        'navigates between the F_Order_line_subform and the F_PC subform
    DoCmd.GoToRecord , , acNewRec
        'adds a new line for each new record entered
    Me.Parent.F_Order_line_subform.Form!ProductID = Me.ProductID
        'Transfers data from one subform to another.
MsgBox "The order of ProductID." _
                & Me.ProductID & vbCr & "has been transfered, Please enter required product quantity to the recent order entry"

End If


Exit_AddToOrder_Click:
    Exit Sub

Err_AddToOrder_Click:
    Resume Exit_AddToOrder_Click


End Sub