Werk #7246: Prevent automatic discovery of duplicate services with the same service description

Hosts using snmp as well as tcp checks may run into a scenario where discovered snmp checks share the same service description as the tcp based checks. One example is the service description Uptime, which is created by the snmp_uptime and uptime check.

In case of identical service descriptions, the tcp based check wins and the snmp based check is ignored.

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