Thank you for reporting. This is a known "bug" or a quirk due to the way it's implemented. Although the UI shows "bookmarked" and "subscriptions" as the only 2 types of community bookmarks, there is technically a 3rd type. The special "home" bookmark. That is why you are technically allowed to both bookmark and make the same community as home.
As for why you cannot remove the bookmark, that is likely a bug. This is probably an edge case where both the home and the subscription share the same community. I'll look into a fix for the next update.