It takes a little digging through the post in the Dash subreddit but it seems that this kind of double spend attempt would be very short-lived - all the other masternodes would cause the blockchain to fork toward consistency and legitimate transactions. https://www.reddit.com/r/dashpay/comments/6x00rl/temporary_disabling_of_instandsend_due_to/
I recently had to try explaining to someone as to why the Zcash blockchain provides provable privacy, where other non-Zk-SNARKs crypto-currencies do not.
The best example I was able to provide to them was in regards to shredding (and reassembling) physical paper documents. They were horrified when the realized that this process is actually made easier via a blockchain, because all of the component parts are public.
In other news …