Discovered that my BA topic has been discussed thoroughly ...after i finished half of it. What to do?
Like @PatriciaShanahan said in her comment, you have the following possiblities:


Check it up if the previous work is the same as the one that you are actually doing. At this point, it would be advisable that you present to your supervisor with your current work and some past articles that resemble what you are actually doing. He could help you to see if its the same or not.
If indeed your work has been published before, then you can make a literature review about it; adding your own experimentation and so on. That could become your thesis for your BA degree.
If  your proposal is new or at least an extension of what has been done before, then you can continue it, but aiming maybe to a future publication.


In all the cases you can, and should, continue with your current work. Remember that what your professors or teachers look after your BA thesis is that you manage to amalgamate what you have learned on your undergraduate period, but not necessarily to present some novel contribution as it happens on the PhD studies.
Good luck!