Steps to reproduce:

1. Create zabbix trapper item
2. Create trigger for that item
3. Turn on media - sending email
4. Receive alerts

Result:

When in the same time you receive alerts - for example 10:00:00 you receive 5 different messages - you will see in mailbox 5 duplicates of the last message.

But in the history messages are different, although have the same timestamp, eg 10:00:00
See screenshot's... - two problems 1, 2 and two alerts 1.1, 2.1
Expected:

Normal alert as in the history