I’m guessing this is not intended behaviour
dropbox.com/s/rjclx6ltvd5mzs0/bug.png?dl=0
Not a bug. That’s the intended behaviour. I’d be happy to hear suggestions for alternative behaviour though.
Maybe just count each cure once. Unless it becomes more effective, but from the price it looks like it could cure 4 times as many people than a cure with this only once. So if the same cure is multiple times it efficiency increases (unless I’m mistaken and a cure @100% will always cure the patient).
I think Multi Effect cures have even more problems. I assume this pill would count as 4 patients treated in the cures screen. It does with different effects, resulting in one sold cure treating 2 afflictions. The only way that would work in real life would be one patient having both afflictions at the same time. Sadly my suspention of disbelief doesn’t survive that many (15-30 per month, depending on production) coincidents. 
(Detailed here: viewtopic.php?f=49&t=11494 )
Regarding same effects stacking, I would cap the effect at 100% and not allow multiple instances of the same effect. It makes no sense to have a cure with more than 100% effectivity. There is still only one person taking it, so only one person should be cured.
I would also disallow the mixing of identical compounds (even in different concentrations or treated by a shaker) in the Multi Mixer. Mixing compound A with compound A would give a larger quantity of compound A with the average concentration.
This particular feature makes the mission The Megacure rather trivial. You just mix and stack your first lvl3 cure and win (assuming it is over 500$ per effect, which many of them are).
I think once you mix the same cure with itself, it should become “super concentrated” or something, and should no longer be mixable. Let that give it a flat or percentage bonus based on the level of your mixer. Stacking a 500$ cure 3 times is pretty easy as long as you have the cash to buy out one of the larger rooms for it, so I don’t think flat stacking like that should be a thing.