We apply zabbix template to the server using the "import" option available in UI. Say, template A is linked with another template B and the templates are applied to the zabbix server.

In the next version/release :
Template A is no longer associated with B
Template A is linked with a new template C
Items from B has been moved to C (C is a superset of B).

Now if I try to import the latest version of the template (A -> C) on top of older version (A -> B) from the "import" option in UI, I get the following error :

Item "<Item_Key>" already exists on "<Template_Name>", inherited from another template.

But, as you can understand, in the latest version of the template (A -> C), template B is no longer there and hence there is no question of conflict between items in B and C.

Details can be found here : https://www.zabbix.com/forum/showthread.php?p=176701#post176701