[xds] ADS reject duplicate resources in grpc_mux

*Title*: in grpc_mux_impl, let's reject a resonse when we see duplicate resources.

*Description*:
see https://github.com/envoyproxy/envoy/blob/main/source/common/config/grpc_mux_impl.cc#L224
We want to not "dedup" but reject the response here when the emplace fails

new_grpc_mux_impl seems doesnt have same issue as it push dup resources to each xds impl, where the dup resources gets rejected.

*Repro steps*:
> send a discoveryResponse with multiple resources with the same name. Envoy accepts it silently.