Extra copies of scheduled reports being emailed

My scheduled monthly reports have been emailed to me as duplicates for a year or more, but I never looked into why.  Yesterday, they were emailed as quadruplets.  Unfortunately, I had the day before effectively stopped entries into ns.log, and the next day I deleted all the additional ns.log files anyway.  I'll turn notification to ns.log back on and run a test to see if I can duplicate the problem; otherwise, my next opportunity to look at ns.log would be the next monthly report run, June 1st.

Any ideas as to the cause?