What causes this issue?

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

System Response

You tried to output two identical messages.
Two messages are regarded as being identical if the message number and
the message class match. For example, messages E122(BRF) and W122(BRF)
in the sense of the action <ZK>Output Message</> (0MS001) are identical.

How to fix this error?

Choose one of the two messages.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.