What does a “Code already in use on another product” message mean?
WHAT/WHY??
This message usually comes up when you are trying to edit/add and then save changes on a product that has no/zero PLU code assigned. However Toniq has found another product that has a Zero as the Manf. Product code or Shortcode already.
HOW DO I STOP THIS MESSAGE??
Go into Maintain Products and type 0 (zero) and Enter in the search field.
The offending product should appear and will probably have a zero in the Manf Product Code field 
OR the Shortcode field.
Either correct or delete the Zero and then F12 Accept Details.
Type 0 and Enter in the search field again and repeat for any additional offending products, until a full ‘normal’ product search screen appears with multiple products displayed.