Steps to reproduce:

1. Create Template with web check named Sample Web check 
2. Create Second Template with web check with the same name Sample Web check
3. Link both templates to one host

Result:

Both templates are linked successfully, but only one web check is assigned to the host.
See screenshot...
Expected:

If two web checks with the same name can not be assigned to the host, frontend must report an error. 

This is realized properly if you try to change already assigned web checks, in that case frontend returns error "Web scenario "Sample Web Check" already exists on host "Zabbix Web". Same behavior is expected when adding templates with conflicting web check names.