Steps to reproduce:

1. Configure an auto-registration action to send notification about new hosts
2. Create an host (i.e. "Server1"), and start monitoring it with agent-less checks, for example icmp ping.
3. At some point, install zabbix agent on the target machine and configure with a slightly different Hostname casing (i.e. "server1") 

Result:
Action gets executed every 1 minute, flooding email recipients.
Apparently it keeps firing because the host registration fails (it already exists with a slightly  differente uppercase-lowercase combination).  
Expected:
Action is not executed at all, as zabbix-server should be able to matche new and existing hostnames using a case-insensitive criteria.