How to reproduce:
From the GUI (php frontend) go to Configuration - Hosts - Create Host
In the "DNS name" OR "IP Address" field put an IP that is already allocated to another existing Host.

Expected result :
Zabbix should split a message that the DNS name or the IP belongs already to a device, eventually asking if you want to continue or not adding the host.

Actual result :
The host is added with no warnings or errors.