This financial document no. is already assigned to financial document &

What causes this issue?

You entered external financial document number &V1&, which is being
used in other financial documents (financial document &V2&, for
example).

System Response

The system issues a message.

How to fix this error?

Check your entry since external financial document numbers uniquely
identify financial documents orgininating, for example, from a bank.

Error message extract from SAP system. Copyright SAP SE.