weird unique constraint on ps_message log sporadically

We have one sqr that runs every day, and most days there are no issues, but every now and then (maybe once a month, but not on any specific date or time), the job will fail and the process scheduler will show no success.  When I look in the out file, I see this:

PRCSAPI.SQC,Update-Process-Status,Update,PSPrcsRqst

SQL Status =     1, SQL Error  = ORA-00001: unique constraint (PSADM.PS_MESSAGE_LOG) violated

in the message log I also see this message:

You are not authorized to run process type PRCSYSPURGE and process name Application Engine. (65,8) P T_PRCS.Schedule.SubmitRequest.OnExecute  Name:SubmitPrcsRqst  PCPC:747  Statement:19