The number sequence for customer invoice under AR module allows duplication of invoice number if manually the 'next' field in number allocation for Customer Invoice number sequence is changed back to same number which has been used (unlike customer invoice voucher where duplication is not allowed for voucher number already used).

The customer invoice can be duplicated in case of manual intervention which can happen in a business.
The system should throw an error and not allow customer invoice number which have already been used.