Two nodes publish to one topic

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There is no build-in priority system in ROS for saying one node has higher prio than other when publishing. So you have to find another workaround.

A simple approach for that would e. g. be to have your nodes publishing on different topics and set up a republisher node that subscribes to both topics and republishes the msgs from the tele-op topic to cmd_vel if there are msgs coming in and otherwise it republishes the msgs from the self-driving topic....