Whenever a journal entry shares the same reference number as a previous journal entry, the message below will appear: The reference number is in use. Please enter a unique reference number.

This will prevent the same number from occurring twice. However, if we want to ignore this error and continue creating those entries, there is a setting you can turn on to prevent this error from occurring. 

Enforce Journal Entry Uniqueness

This setting is located in the system settings and can be turned on by Centerbase admins for the site. Click on the gear in the lower left hand corner and click on "System Settings". From there, go to System Configuration > Advanced Configuration and find "Enforce Uniqueness". There will be an option called "Require Journal Entry reference numbers to be unique".