Problems sending duplicate data on publisher PC

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See #q50323 for an old related question.

See ethzasl_message_transport for an old, currently unsupported, set of alternative transports that offer UDP multicast for ROS 1 that would address some of what you ask. Note the limitations on maximum message size (the implementation does not support fragmentation and reassembly).

As for ROS 2: the question would actually be: "does DDS send messages to remote hosts multiple times if there are multiple participants running on the same host?", as ROS 2 primarily uses DDS as the underlying middleware.

I don't know of any ROS 2 support at this time, but some DDS vendors do support UDP multicast, such as RTI (docs here).

Edit: a workaround in ROS 1 could be to have only a single subscriber register for the topic published by the remote host and then have that single subscriber republish any incoming messages locally. That would avoid multiple datastreams between the remote host and the subscribers. It would introduce computational overhead though (although it could be relatively low, when using ShapeShifter fi). Perhaps topic_tools/relay could be part of that workaround.