If a service provider wishes to implement a certain interface multiple times (e.g. parametrized by available hardware or software at installation time), it can't provide them all (unless they have different implementation versions, but that is a hack).

Basically each interface will have different QServiceInterfaceDescriptors (e.g. operator== returns false, since it considers custom attributes), and these descriptors are passed into the service's createInstance methods, so it's just the XML parser that decides that this is not allowed (since it only checks interface name and version, and implementation version).