Given the fact that all platform installers only support one instance of Jenkins I'd call that the default assumption. While it's not "mandatory to have [at most] 1 instance", it's just likely to be not as well supported.
In this case, it seems to be more of a diagnostic message rather than a real problem (as you can see in the source code linked above) for the unusual situation that one service (Jenkins) is advertised multiple times from the same host.
Start Jenkins with java -Dhudson.DNSMultiCast.disabled=true ... -jar jenkins.war --httpPort=... as documented in the wiki to disable advertising the service this way. Or are you using DNS Service Discovery in some way?
Seems you have misconfigured the network loopback.
Useful links: