According to the docs it *should* be unique, so in that case the only thing missing according to you is a unique constraint on the database.
Personally, I make the key unique myself by using it in a 'compound' way. I used a license plate number, followed by a colon and a date (assuming two insurance claims were not started on the same day). In other demo I used the insurance claim number for the businesskey, making it unique in that way and prevent duplicates. I've not run into a problem with this, but it might not hurt to add a unique constraint. Please file a jira issue for this, though I'm not 100% sure what the outcome might be. You could discuss this on the dev list first.