it's as rare as hens teeth. You are already very lucky to have the opportunity, i hope the luck continues and the questions are on your side.
On how it differs - presumably there was a calamity last time, so hopefully all will go well this time. Other than that, it shouldnt be massively different to the process that you had before.
When you finish it though, you might want to be very explicit with the testing people to ask how the score is shared with the unis, and if its the same process as they use for everybody else or if you have to do anything different.
Good luck.