Duplicates In Table/Query

I am using a combo box to assign gun numbers on an employee form (frmemployees). I also have a query listing only the gun numbers that have been assigned. When I select a gun from my combo box, I what to evaluate if it is already on the gun assignment query. That way I cannot assign the same gun to two different people. I am using the before_update event to do this. The code that I am using is as follows:

Private Sub intgunNumber_BeforeUpdate(Cancel As Integer)

If DLookup([intgunNumber], "qrygunassignmentcurrent", Me.intgunNumber) = Me.intgunNumber Then
MsgBox "Item # is already listed.", vbOK, "DUPLICATE FOUND"
Cancel = True
End If

"me" represents my frmemployees form. The problem is that it's causing an event even for the numbers that aren't listed on the query. I am trying to get the evaluation correct but am having difficulties. All the fields are numeric fields. :( :mad: