Answer: By default ExpressMaintenance allows you to have duplicate PO numbers in the purchase orders. However, you can limit this by going to Administration / Defaults / PO Defaults. Select the option Warn on duplicate POs and change the value to Yes or No as desired.
In addition, you may need to adjust the database table constraints to allow for duplicates. To do this, you can use the option under Utilities / Run SQL Statement or you can use ExpressSQL or some other SQL program to execute the following commands.
Drop the existing Index with does not allow duplicates:
alter table porders drop ix_porders
Re-create the index without duplicate contraint:
CREATE NONCLUSTERED INDEX ix_porders ON porders(Numbered)