Create host with one item
Create template with discovery rule with item prototype
Make sure that item and item prototype keys are the same

Try to link template to host

error:

Created: Discovery rule "terdisc" on "thost".
Error in query [INSERT INTO items (type,snmp_community,snmp_oid,hostid,name,description,key_,delay,history,trends,status,value_type,trapper_hosts,units,multiplier,delta,snmpv3_securityname,snmpv3_securitylevel,snmpv3_authpassphrase,snmpv3_privpassphrase,formula,logtimefmt,valuemapid,delay_flex,params,ipmi_sensor,data_type,authtype,username,password,publickey,privatekey,flags,filter,port,inventory_link,lifetime,interfaceid,templateid,itemid) VALUES ('0','','','10168','terdiscitem','','agent.version','30','90','365','0','3','','','0','0','','0','','','1','',NULL,'','','','0','0','','','','','2','','','0','30','10114','26322','26325')] [Duplicate entry '10168-agent.version' for key 2]
SQL statement execution has failed "INSERT INTO items (type,snmp_community,snmp_oid,hostid,name,description,key_,delay,history,trends,status,value_type,trapper_hosts,units,multiplier,delta,snmpv3_securityname,snmpv3_securitylevel,snmpv3_authpassphrase,snmpv3_privpassphrase,formula,logtimefmt,valuemapid,delay_flex,params,ipmi_sensor,data_type,authtype,username,password,publickey,privatekey,flags,filter,port,inventory_link,lifetime,interfaceid,templateid,itemid) VALUES ('0','','','10168','terdiscitem','','agent.version','30','90','365','0','3','','','0','0','','0','','','1','',NULL,'','','','0','0','','','','','2','','','0','30','10114','26322','26325')". [CHost.update -> CHost.massUpdate -> CHost.massAdd -> CTemplate.massAdd -> CTemplate.link -> CItemPrototype.syncTemplates -> CItemPrototype.inherit -> CItemPrototype.createReal -> DB.insert]