This is as old as the history of education. We can choose to get the kids to work at the right level for each kid or we try to get them to be ahead of others even when they are not capable. Then we repeat them later to give them advantage of familiarity.
Behind the scene, there is science to this. Researchers have found out that human short term (working memory) is very limited. We can only ever focus on 4-5 things at a time. The 'chopping block' in our mind only has room for 4-5 things at any time. However, anything that we already know in the past could be recalled and swapped on and off this chopping block (working memory) rapidly and automatically without putting stress on our mind. This is called automation of memory. So if you have done it before (even badly), the familiarity will help you become more confident and faster next time around. This is what they called "practice makes perfect". It works for learning too. But it may NOT work for really difficult stuff that won't even stick around in memory if you never manage to understand anything at the first place.
Relating this to facts on this system, there has been an example of a strong performing kid who repeated the selective program. The parents pretended that the kid was in year 5 and put her on selective program. Then when she got to year 5, repeated the selective program again. She scored ok with unstable scores the first time. She scored extremely high the second time though same stuff (not sure if this is memory or real learning). She beat most kids who were lower than her in OC profile and beaten by kids with higher OC profile (who did not go through a repeat).
So at the end, it made no difference. However the advantage could be the predictability of getting a good result. The disadvantage was the frustration at the beginning and the awful waste of time that could have been better spent for other things (such as a good childhood, other skills and bonding with parents and siblings ...)
Teachers always recommend working at the right level. Yet, sometimes, kids have to jump to try to become familiar with the difficulty of OC and selective tests so they have some chance in the real tests. This is a strategic choice based on the personality of an individual kid. There is no clear right and wrong answer. However, one thing is clear, if a kid works at slow pace and improves the scores steadily towards 90% and reaches near the end of each program, high success is guaranteed. Working at a moderate pace this way will give the kids more time to do other things. As the result, all 3 scholarship winners this year were either not tutored or lightly tutored. They had time for music, sport and other activities. This was partly why the private schools picked them over the "academic only" kids.