--> In Option --> GL 
--> Check the “Warning message for bookings with the same voucher numbers” checkbox.


--> Create a single booking with the same voucher.
--> Received a message indicating that the voucher already exists (message shown below).


YES --> The overview shows the suggested voucher.


NO --> Saves the same existing voucher.


CANCEL --> Allows manual changes.


Once you check the "Always use the next available voucher" checkbox and click YES number will automatically update to the next suggested number.