A duplicate execution request is found for program?

I have a few clients that aren't running a task sequence and the error is "A duplicate execution request is found for program".  I searched this forum and one suggestion is that maybe two advertisments are trying to run the same program.
So, I checked but didn't find any duplicate adverts for the task sequence.  So here is my question:
How can I delete the policy for the task sequence advert from the affected client, so that it can start clean? 
 
Thanks in advance,

It's hard (maybe super close to impossible, although I rarely use the word impossible) to find and surgically delete an individual policy or, because it's a TS, set of policies.  As an experiment, what might be easiest is pick on a client, and using Client Center from the pull down try "Soft Policy reset".  If the behavior still isn't resolved, then try the "Hard Policy reset".
Can't hurt, anyway.

Maybe it hasn't forgotten an earlier advertisement you've tasked it with that hasn't completed yet? Maybe look in hklm software microsoft sms  in the distribution history (under wow6432 if 64bit ofc).