When a given interface respects the cross posting method, it doesn’t show multiple posts, it shows one post with a list of places it was cross posted to.
Unfortunately, not every interface respects that part if federation. Most lemmy apps don’t (afaik, none do, but I can’t claim to use all of). If kbin via web isn’t following that, it isn’t that person’s fault.
Now, that’s different from someone making multiple posts of the same thing because they don’t know how to cross post lol.