Invoice document & / & already checked again

What causes this issue?

The invoice document was automatically verified in the background by
Batch Invoice Verification for a second time or changed in a second
session by you or one of your colleagues.
The changes you have made in this (older) version therefore cannot be
saved and will be lost.

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

How to fix this error?

If you have changed further invoice documents, save these on the
invoice verification comparison overview screen so that these changes
will not be lost.
Refresh the screen. A list of the most up-to-date data on the selected
invoice documents appears.
Check the invoice documents that you were unable to save and make the
changes again.

Error message extract from SAP system. Copyright SAP SE.