Billing document request for reference xxx already exist.

We are unable to create the new billing document request (fixed price method) and getting an error that " Billing document request for reference 1000000001 (0000000000) already exists. From the app 'Manage Project Billing' we select the billing element, and choose prepare billing. New project billing request screen displays but BDR request fails with the above mentioned error.

We have not created any BDR for this billing element yet!