Hello,
Then you should validate your logic in the custom exit with help of a ABAP consultant.
If two customers have same first few digits then this would be a problem.
The best way to handle this is, to define a logic which will never create duplicate serial numbers.
Additionally the temporary work around would be to manually change the serial number to make it unique in CO02 transaction and then release the production order. I think having duplicate serial numbers prevents order from being released due to errors (Confirm if this is right).
You can find exit using below helpful links, also to debug you can enter /h during order release if your serial number is getting created on order release and put a breakpoint at statement "CALL CUSTOMER-FUNCTION".
Serial Numbers: Customer Exits
User Exit to Modify the serial Number in Produc... | SCN
Best Regards,
Ameya B.