Clearly the chances of more than one car entering the same block are statistically tiny, given how rarely it happens compared to the millions of rides taken around the world every day.
Multiple redundancies are required to further reduce the likelyhood of a chain of events coming together.
My humble Honda has anti lock breaking, electronic break distribution, dual circuit breaking system, etc. These reduce the likelihood of an accident. It then has seatbelts, side impact bars, airbags, crumple zones, etc to provide protection should an accident still occur.
There will always need to be an acceptable compromise between safety, likelyhood and cost. I'm sure rollercoasters are safe because they allready have multiple redundancies and perhaps in the future they will have more.
I believe that Merlin will do the right thing and do whatever needs to be done to prevent a recurrence.
I want rides that are SAFE, but have PERCEIVED danger.
I hope that it will always be 'Fright Time' with 'The World's Most Intense Ride Experience'.