There is already a session running for the copy specification JOBNAME. The session will abort.

Issue:
The job shows the following error in the log

There is already a session running for the copy specification <JOB NAME>. The session will abort.

The job cannot be seen in the monitor tab of Data Protector manager.

This is known to occur in HPE Data Protector 9.08 but it's likely to be the same for other versions.

Cause:
The job has failed but the csm.exe process that controls the job appears to have crashed and been left running.

Resolution:
Find the running csm.exe for the job and kill the task with task manager. 

I found the task by looking at the CPU Time column to see which one had been running the longest as there was a 2nd task running. The csm.exe that had crashed was using a large amount of memory (>5 GiB) and maxing a single core on CPU.

To be certain you have the right csm.exe process, it is possible to find the environment variable DATALIST by using procexp.exe to look at the process properties.

Once the broken csm.exe has been killed, the job should run normally.