If you enable
eperson.subscription.onlynew = true
not all new items are sent during subscription, only those not changed.
This prevents notification about items, which are new but have been
changed. This is quite often the case e.g. as the metadata language
qualifier is hardcoded any varying language qualifier has to be changed
post submission via edit item.
The alerting service should be based on date.accessioned or date.available
in contrast to lastmodified which harvest uses.