I'm sorry I misunderstood the difference between a subscription and a subscription "event." I wish understood better what an "event" is with regard to SSRS subscriptions. I'm having trouble finding documentation on the AddEvent stored procedure. It seems that under "Subscription Processing Options" there are two options:

When the Scheduled Report Run is Complete

On a Shared Schedule

I interpret your example as using the first option, and that the AddEvent method is setting the schedule as if the "Select Schedule" button had been pressed and the user selected a date/time to process the report. If that's the case, then there is only ever one "When the Scheduled Report Run is Complete" event, and any time you run "AddEvent" it automatically deletes the previous schedule. I would expect that if you used a "Shared Schedule," the schedules would accumulate - but perhaps "AddEvent" doesn't pertain to shared schedules. Again, more documentation from Microsoft would be helpful.