The set is in sale, and I already bought one of the books that makes the set.
https://www.logos.com/product/5309/reformed-dogmatics
I bought the first book of Bavinck's Reformed Dogmatics set when the whole set was not on sale.
Now it's 25% off, I would like to buy it, for sure - but I am not sure what will happen as I have the first one.
Are they considered separately?
Also, is experts@logos.com always this late to reply?
I asked the same question and I haven't received an answer yet 
Thank you!