Help me with this.. I dont really have that much knowledge about excel...

I have a Record a Record that is in the Pivot table like this:

Item Code Serial
----------------------------------
Ques | 23120 | 00112255
| | 00112233


what i want to do is I want to inform the user that
one "Code" has multiple "Serial"... how will i create a macro
that will alarm the user that there is a duplication.

Thanks and more Power...