Steps to reproduce:

1. 
   Create a template with an LLD rule that creates host prototypes
2. 
   Configure a host prototype with a "Visible name"

3. Link the template to a host

4. Let discovery run

5. 
   Unlink the template from the host (do NOT use "unlink and clear")

6. Link the same template again (or an updated version with same keys)

7. 
   Observe that the number of host prototypes is duplicated
8. 
   Repeat steps 5–6 and observe duplication (triplication, etc.)

 
The problem is reproducible with any discovery type.
Tested with: * Simple checks

• Zabbix internal

The issue was first noticed in official template "VMware" version 7.4, but it is also reproducible in a custom template recreated from scratch.

 
Important note: The issue ONLY occurs when "Visible name" is set in host prototype. If visible name is empty, host prototypes are not duplicated.
Result:
After re-linking the template, duplicate hosts appear as result of the same discovery rule.

Attached screenshots show: # Two discovered hosts generated from the same LLD rule

1. Host prototype configuration with "Visible name" set

 
Visually the hosts appear identical and no differences can be spotted in the host list, but internally the discovery rule reports an error due to duplicated host prototypes.

Expected:
Host prototypes should not be duplicated after template re-link.