Steps to reproduce:

1. Define new action for Linux  host auto registration with next conditions: "Host metadata contains Linux" and operations: "send message to user groups", "add to host group: Linux Host", "Link to template: Template OS Linux"
2. Install zabbix agent on linux host and define next param: 
   1. ServerActive=[your zabbix server]
   2. #Hostname=Zabbix server  // disable this line
   3. HostnameItem=system.hostname
   4. HostMetadataItem=system.uname
3. Run zabbix agent and wait for auto registration message
4. Change server uname (for example upgrade Linux kernel)
5. Restart zabbix agent

Result:
After kernel upgrade,  will get auto registration message again, but no new host created in zabbix.
No log about new host creation.
Same message, after each kernel change.
Same behavior with Windows hosts.
Problem appeared after update to from 3.4.15 to 4.0.1
Expected:
Only one message, after first host registration