It's not really a great term but, at the same time, calling it fraud--which it sort of is in some contexts--feels kinda strong.
If your goal is to share information, you want quality articles in multiple places so that people can get to them. If your goal is to use journal publishing count as a professional metric, then double publishing becomes unethical.
The modern equivalent would be to declare it unethical to share an article on both Reddit and HN.