Yes, pretty much every serious venue is considering only papers with novel, unpublished research, so if you've published "that thing" already somewhere else the

Yes, pretty much every serious venue is considering only papers with novel, unpublished research, so if you've published "that thing" already somewhere else then that disqualifies you from getting a well-ranked publication out of that piece of research, and it's strong disincentive to publish anything outside of the venues that 'count' for the purposes of evaluating you or your project goals.