Hi,
There might be some issue like this way.
1:- it won't happen two documents have single number. It may happen it is not updating the data table , like VBRP, VBAP, VBFA etc..
2:- If any user exist activated for billing document ( RV60AFZZ include and exit is Userexit_number_Range) where there is wrong data maintained in the ztable for your user exit.
Check with your ABAPer if user exit is active , if active then check the table entries with t-code SM30 .
Hope it will help you to understatnd.
Regards
Pitabash