Steps to reproduce:

1. Create host item with type = Zabbix trapper
2. Create trigger for this item with formula (count(/testhost/testitem,#2)=1 or change(/testhost/testitem)=1) and nodata(/testhost/testitem,30s)=0 and event mode generation = Multiple (basically i want to create separate event for each value i received)
3. Populate value with data (for example using bash on zabbix server): for thing in {1..10}; do ((i=i%N)); ((i++==0)) && wait; /usr/bin/zabbix_sender -z localhost -s testhost -k testitem -o $thing & done;

Result:
After step 3 we will get 11 events instead of 10
Expected:
After step 3 we get 10 events

The issue is probably in how nodata() works – it recalculates every 30second and at the time of recalculation new event is generated.

The reason trigger expression is configured like that – i want to create new event on each received value. Count is used to escape in case we didn't have any values in item before (eg new host or it history was cleaned by housekeeper). Nodata is a way to close old events and (not to create new ones until we get new value). Attached is host export to ease of testing.