If I might ask, which SMP client are you using, Deino or MPICH? Have you updated the .exe to the 6.24 version from
here? I'm just running through ways to keep your client up to date, sorry if this sounds elementary.
EDIT: crap, I just remembered: after you delete the core you also need to delete the queue file and work folder as well or it will just chuck you another of the same WU. That's probably why that happened.