Yeah, this is a huge limitation of the beta program. (We call it "boosting blind" -- the nominators have no way of knowing if a story has already been Boosted, unless the story is in their publication.)

We're working on it, but the engineering team has been VERY busy with the Medium Partner Program work, and so Boost functions just have to wait their turn. Hopefully by the end of the year, things will be out of beta!