Boosting re-sends the original message, with the original message id attached, and both Lemmy and mbin filter filter out duplicates. On Lemmy, upvoting a post boosts it, and on mbin the functions are separate. Boosting works to get the community/magazine group actor to re-send the post to subscribed remote sites, so if the site you're using subscribed to a community after the original post was made, it could now receive it thanks to the boost.