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To date, I've only ever seen this in the mergeEvt task.  This is symptomatic of a problem upstream in the doChunk streams (running the makeEvt task).  In every instance, I've found one or more of those makeEvt tasks had a multiple submission.  Rolling all the doChunk streams back with the multiple submissions fixes the problem.

Set up a rollback when jobs still running

Sometimes you need to roll back a stream but there are jobs still running that prevent the rollback from occurring immediately.  Warren provided this little tidbit to set up the rollback and not have to keep checking back:

When a run is not ready for rollback, I usually do this:

$> del=<delivery #>

$> until ~glast/pipeline-II/prod/pipeline rollbackStream --minimum --force L1Proc[$del] ; do sleep 300 ; done

(in bash) so I don't have to keep checking.

I haven't actually used this yet so I'll update it (or remove this line) when I can verify that it works.