Is this by design, or a bug?
Most of the time, when I see one entry in both categories, and I review one, the other is gone when I revisit the review overview just afterwards.
However, I have experienced being able to review the same post in both categories. This does not happen often: On about 1500 reviews, I can only recall this happening twice.
Is that by design? or Bug?
Without having direct access to the site's code, it is impossible to know whether it is design or by accident. It can be design (for instance, a question may be re-injected in the queue if it is edited, or it may just be a bug. (Personally, I don't care.)