This is most likely harmless. Check comment from Tom Jenkinson:
Yes, I agree that they are harmless. BUT, 200+ WARNINGs per day and 10-20 ERRORs does clutter up the log and we watch the log pretty closely to make sure the system is running correctly. Just wondering if there is a setting somewhere to at least suppress the WARNINGs.
I found bz which seems to be reporting the issue you have:
Problem is that transaction manager is calling rollback twice for the same xid. Second call causes those ERRORs/WARNs to be thrown to log. Still this appears to be harmless.
I guess that we can live with it.