Keeping Focus On Combobox?

I have 2 comboboxes (cboTeacher1 and cboTeacher2) and I do not want them to contain the same value. If the same value is chosen, I want a messagebox to show the error and keep the focus in the offending combobox.

I wrote the code below for cboTeacher1 and it functions in the case that your are making a selection in cboTeacher1; however, if you click ok on the msgbox and then press tab, the mesage box is not redisplayed and the focus is switched from cboTeacher1.

I tried placing the same code also in the cboTeacher_Exit event and when tabbing out of cboTeacher1, the msgbox is displayed, but the focus will not stay in cboTeacher1, it shifts to cboTeacher2.

How do I keep the focus locked in cboTeacher1 until both cboTeacher1 and cboTeacher2 contain different values.

Private Sub cboTeacher1_BeforeUpdate(Cancel As Integer)
If cboTeacher1.Column(1) = Me.cboTeacher2.Column(1) Then
' Both Primary and Secondary Teachers are the same
'Determine if either Teacher cbo contains a null value
If IsNull(cboTeacher1) = False Or IsNull(cboTeacher2) = False Then
' Alert the User of the discrepancy
MsgBox "You can not have the same teacher's name in bowth Primary and Secondary Teacher"
Me.SetFocus
End If
End If
End Sub


Thanks