I don't know what the stats actually are. But all I would say is practice and prepare as best as you can. I did much more prep second time round, especially everyday in the week leading up to it as you know it's your last opportunity and I passed. Are you about to do a second assessment soon or hoping to get the chance? If so best of luck.
The overall pass rate is around 40% for the Assessment Center or at least it was 4 years ago. I dont know how the pass rate differs between first time and second time. I passed it second time. First time I only failed on the structured interview with the CCL assessor at the end, despite passing the computer and written tests with ease. Second time, I just managed to struggle through it with a borderline pass in the structured interview, but once again I passed the computer and written tests even more easily than the first time round. I would think the pass rate would be slightly higher for people going through their second attempt as I found it much easier the 2nd time. It feels like there is slightly less stress on the 2nd attempt because you know what to expect, while on the 1st attempt is stressful as I went in expecting it to be harder than it actually was.
I don't think having a 2nd attempt at the tests affects the overall pass rate because its down to personal preference. I.e. If you do lots of prep and practice then you have equal chance no matter how many times you take the test.
Generally if people fail say the Group Bourdon test the 1st time round - they will fail the same test again the 2nd time round.
Also, it's worth noting that some companies do additional tests such as Heathrow Express and South West Trains so you may get "new" tests to do in addition to the core ones if you apply to a different TOC.
However, if you do the tests a 2nd time round is good to mention in the Driver Manager's Interview as it shows motivation (you suffered a set back but didn't give up etc....)