ACTION_DUPL_EXISTING_KEY test failed for '&1'

What causes this issue?

The test executes an action on &V1& for multiple instances with same
existing key.
One of the following could be the reason for the failure of the test:
a) Action key not listed in 'RESULT'.
b) Instances found in 'FAILED'.
c) If there are keys listed in failed for these keys there is no error
message.

System Response

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.