In the before_update Event of the field enter the following code:
Private Sub yourfieldname_BeforeUpdate(Cancel As Integer)
If DCount("yourfieldname", "yourtablename", "yourfieldname = '" & Me!yourfieldname & "'"

> 0 Then
MsgBox "Duplicate ID number, please insert a unique ID number"
DoCmd.CancelEvent
End If
End Sub
HTH
Dave
This will loop through and check for a unique id number and will return to the field until a unique number is entered.
As an aside, you can put the following code in the 'Event' event of the field to remind users that it requires a unique ID number
Private Sub nameofyourfield_Enter()
MsgBox "You will have to enter a unique product number in this field. If the product number is a duplicate, the database will reject it and wait for another input."
End Sub