Node::Advertise doesn't fail when advertising a duplicate service

[peci1]

I'd expect that there can only be one "responser" for each advertised service. But that's not true.

Taking code from the examples:

$ ./responser_oneway &
[1] 10281
$ sleep 5 # to be sure the first responser gets properly advertised
$ ./responser_oneway &
[2] 10313

$ ign service -s /oneway -i
Service providers [Address, Request Message Type, Response Message Type]:
  tcp://172.17.0.1:39721, ignition.msgs.StringMsg, ignition.msgs.Empty
  tcp://172.17.0.1:43201, ignition.msgs.StringMsg, ignition.msgs.Empty

$ ign service -s /oneway --reqtype ignition.msgs.StringMsg --reptype ignition.msgs.Empty --timeout 2000 --req 'data: "Hello"'
Request received: [Hello]
Service call timed out

So it seems the discovery service is perfectly fine with having multiple providers of a service, but the client gets confused (the console only has one Request received... line, which means only one of the service responders got called). The timeout is weird.

I think it's wrong that the discovery service registers the other responser (but given the distributed nature of the system, maybe it' s inevitable?). However, Node::Advertise should definitely check for duplicate services before returning true.