I agree with Asriel and Bombur, Pre-bonding is so very important. I know it can be a lengthy process and people get impatient but it will be worth it in the end when you have a bonded pair of bunnies. I had the same issue with my bunny and a bonded pair that my sister has.
Pre-bonding could be just switching their pans around, so that Pie has Mango’s litter box or cage tray and Mango has Pie’s. I would highly suggest and urge you take your time and not try to rush the process. Lots of people overlook the pre-bonding process and just want to dive into it, i did before Asriel informed me that i was skipping a step! 
Bunnies are naturally territorial and Mango is now in Pie’s territory. So Pie will get upset with an unknown bunny in her space, so getting them used to each other’s scent would help a lot because then Mango becomes a familiar scent to Pie and it wont make her prone to getting aggressive.
And always start out small, and then get bigger. Sessions should be small at first.
When you first meet someone you aren’t going to get married right away, and when bunnies see each other they won’t either. That how i liked to think of it anyway! lol 