No duplicate check could be made for class &1

What causes this issue?

For the billable items of class &V1& to be transferred, no duplicate
check could be made because at this time another duplicate check by a
parallel process was active for the class.

System Response

To prevent the transfer of possible duplicates, all billable items to be
transferred were saved unchecked in <DS:DE.BITSTATUS_KK>'raw'</> status.

How to fix this error?

The raw items can be processed further in a downstream transfer process.
The duplicate check is repeated automatically in the transfer process.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.