Deetah
the main issue for stamp duty is where you are changing the title on the property. You dont give huge details, but if what you are saying is that you are on title and the other is not, and you are buying the other out, then it looks like there would not be stamp duty as there is no change in title. Is this what you are saying?
Problem for giving any advice is knowing all the facts, so as for all these matters, if you need to rely on well meaning views (meaning myself), it always on the basis that you get professional advice if its a significant matter.