problem with key in item prototypes in discovery in template

[peter68]

I have ca 50 switches and i try to add to template OID with cpu load.
I'm not sure about "Key" parametr, i try many options:

["{#HOST}"]
switch_cpu.["{#HOST}"]
switch_cpu.["{#SNMPINDEX}"]
switch_cpu

but i still get errors like this:

Cannot update item: item with the same key "switch_cpu.["{#HOST}"]" already exists

Rest of my item prototypes works correct in templates. When i put this oid in host configuration - it works but when i try put into template it fails. What should be correct "KEY" parameter ?

my config:
configuration -> templates -> discovery -> item prototypes -> switch_cpu

name: switch_cpu_$1
type: snmp v2 agent
key: switch_cpu.["{#HOST}"]
oid: 1.3.6.1.4.1.89.1.8.0
rest options are correct because they work for other itemp

thanks for help,
Peter