Steps to reproduce:

1. Upgrade Zabbix 5.x installation to Zabbix 6.x installation OR import Zabbix 5.x template. For example if testing with Template OS Linux SNMP template. The following Zabbix 5.0 templates are needed:

• Template OS Linux SNMP
• Template Module EtherLike-MIB SNMP
• Template Module Generic SNMP
• Template Module Interfaces SNMP
• Template Module ICMP Ping

1. Download Template OS Linux SNMP for Zabbix 6.0 LTS
2. Try to import the updated Zabbix 6.0 LTS template to update the older template.

Result:

The given error: 

• Item with key "icmpping" already exists on "Linux by SNMP".
  Maybe this error is expected as in Zabbix 5.0 the Template OS Linux SNMP has many template links but since Zabbix 5.2 the template links seem to have been removed. However unlinking all the templates does not solve the issue.

Expected:
Template items updated without any errors.

Tested 1:

• Unlinked (without clear) all the linked templates to "Template OS Linux SNMP" in Zabbix 5 template. See screenshot:
  
• Tried to import Zabbix 6 version of the same template (with all update/create checkboxes selected only)
• Same error is given:  Item with key "icmpping" already exists on "Linux by SNMP".
  

Tested 2:

• Unlinked (without clear) all the linked templates to "Template OS Linux SNMP" in Zabbix 5 template
• Tried to import Zabbix 6 version of the same template (with all update/create checkboxes selected, except without create: Items, Discovery rules, Triggers)
• It gives a pile of errors... See screenshot
  

I have expected that the import should have updated `icmpping` instead of complaining that it already exists.

The only way to import the template is to use the "Delete missing" section. But this cause all items to be removed from all hosts and history being lost. This is obviously unacceptable.