AHA! NOT a Bug?
The new modules does not "upgrade", they "replace".
So THAT is why you are allowed to buy multiple of the same thing, because it COULD be better than your current one.
IMPROVEMENT:
Some kind of color or other marking on eg. the item border, to show that it is better or worse than your current one, and if you do buy it, maybe add a pop-up warning, just to confirm that the player indeed intends to buy a worse item.