I would be furious too and would have gone to the class teacher straight away if I found out this was happening. And then on to the head if they didn't listen. I would also have dragged as many of the other 9 parents along to complain at the same time so I couldn't be fobbed off that I was the only one unhappy at the situation.
They are discriminating against these children because they have had to repeat a trip they had already done and it sounds like the other kids are doing 2 trips and not one like your child.
I would speak to the teacher first thing tomorrow morning (if aquarium trip is not tomorrow morning - or even if it is) and demand ask why the children are being denied the aquarium trip. And then very nicely demand that they go on it.
If they say no, suggest that to make it more fair to the Y1 children that have been in the R/Y1 class who have not yet had a chance to go on a new trip unlike the others, that they swap your 9 with 9 of the other Y1 children so that at least they get to go on a new visit. If they say that this isn't fair on the 9 children that won't now be going on the aquarium trip then point out that exactly - and it is even more unfair on those children that aren't going.
They might try to throw practical issues in to stop them from going - such as not enough space on coaches or child:staff ratios. Would you and the other parent be able to go and help be a chaperone at short notice? Could they get a minibus or couple of MPV taxis or parents with big cars to help with getting them there? Try to think of all these sorts of issues in advance and have solutions to them so that if they try to use them to stop the kids from going you have a comeback to all of them.
The thing is - whether they like it or not - at that age, school trips are massively exciting and important to the kids on all sorts of different levels. Your 9 kids have already missed out on being in the 'real' Y1 and so will be feeling a bit off about that - to miss out on the trips that are deemed important enough for their contemporaries to go on is denying them the chance of doing something that is relevant to their Y1 work (and if not, why are the others going?). Plus it means that they did a 'baby' trip and not the 'big boy' trip which will feel a bit like a kick in the teeth or could be used by the others to tease them or come next term they won't have the experiences that their other classmates will have had and might want to use them later on.
All going on a Y1 trip together would have validated that they really were in Y1 and not that they were doing R again (especially given they were repeating the trip).
As a last resort I would demand ask nicely if the 9 children could be taken on the two trips near the start of the autumn term (or later this week/next week) so they will then be on a par with their classmates. or taken on the first trip if you manage to get them to go to the aquarium this week...
it is worth saying something - just because the classes are going to be different this year, doesn't mean something like this won't happen again. And it is your child that has already missed out on a trip that you yourself think they would have benefited from and might miss out on another one. The staff might have just been lazy about sorting it out and figured that if nobody says anything then they have got away with it or it could be that they just were not thinking straight and just thought each class would go on a trip with their teacher, forgetting that would mean some children repeating the reception trip.
Oops. just realised this has become a bit long but hopefully it has fired you up to go and get your child (and his classmates) on the second trip! and get taken to the first trip location too... good luck!