There are a couple of different ways.
1) Set field validation to be unique. You can set a custom message if validation fails. In File > Manage > Database, click the field, click Options, Validation tab.
2) Set up a relationship between the table, and a copy of the table (another table occurrence) based on the PO #. Set a calculation such that Invoice::PO# <> InvoiceSelf::PO # as part of a Script Trigger. (I would probably use OnObjectModify.) If that happens, show the dialog.