Firstly, AtticusFinch, shut up. This is a place for constructive comment, not frightening people who are uncertain and come for help.
On topic, the actual quote from the syllabus (p.26-7) is:
The Board of Studies said:
the topic: ...
- may be developed from a case study but should cover substantially different
ground
- must not overlap significantly with the examinable content of any other School
Certificate or Higher School Certificate course.
So technically, it doesn't matter whether you're studying it or not, you're not meant to do it. I suspect Abbeygale is right in saying "The overlapping rule seems to be to prevent us using information and resources we've gained in class in our MW.", but I think it's meant to also cover the fact that there will be lots of resources out there for other case studies too (eg. on places like Bored). It's designed to get you thinking about historical inquiry and rewards imagination or interest in history.
That said, if your teacher has no objection to a particular topic, then you will have no problem in doing it. They are
not forced to send your essays to BOS after they are marked; there is a competition you can enter if you wish.