Use the BeforeUpdate (if you want to cancel it) or AfterUpdate (if you just
to to notify the user) of the control. Use DLookup() to check for a
duplicate in the table.
This kind of thing:
Dim strWhere As String
Dim varResult As Variant
With Me.City
If IsNull(.Value) Or (.Value = .OldValue) Then
'do nothing
Else
strWhere = "[City] = """ & .Value & """"
varResult = DLookup("ID", "Table1", strWhere)
If Not IsNull(varResult) Then
MsgBox "Duplicate"
'Cancel = True
End If
End If
End With