You can set Transaction Number Series. To do this you can go to Module>>Nominal Ledger>>Settings>>Transaction Number Series. By filling this in and then using Access Groups to limit the number of people who can access to it, you can prevent this issue from happening in the future.

Please see the link below for more details:
http://www.hansamanuals.com/main/english/mailnumber___5816/theconf___535/manuals/levelzero___5816/version___62/hwconvindex.htm

HansaWorld Support Team