Financial object '&' already exists

What causes this issue?

You attempted to create financial object '&V1&'. However, a financial
object already exists for the specified key of the original object ('
&V2&', internal number '&V3&', external number '&V4&').

System Response

You can only create one financial object per original object. The system
changes financial object '&V1&' instead of changing it.
Online transaction
Check the keys of the original transaction you entered on the initial
screen.
Data transfer
Check the data in the sender file, in particular the keys for
identifying the original transaction.

Error message extract from SAP system. Copyright SAP SE.