split out from ZBX-2152.

a) we should document a bit more about duplicate engineids, how to debug that etc;
b) we could try to detect such a problem and provide better error messages, too. quoting asaveljevs :

My research shows that NET-SNMP provides information about the discovered snmpEngineID. I propose to remember what snmpEngineID sits on what IP:port, and if we discover a collision, we provide a good log and NOTSUPPORTED message to help identify the problem. I do not think we should do anything beyond that.