I hope you got to removed it in time. If you had not, not only would the discount be combined with your long term discount, but apparently AirBnB does not know how to properly combine percentages.
To combine two percentages, they just add them together. That's what happened to me. I complained to customer service, but if you can believe it, they defended it saying that it was correct and I should set a different long term discount if I don't want to discount so much.
I'm only happy they didn't combine 50% discount with 50% discount or I'd be getting 0 according to their math!