Avoid duplications inside nested repeat groups

1. What is the problem? Be very detailed.
Hi. I know that the issue of duplicate values inside a repeat has been already discussed and properly solved. But now I am facing a kind of unpractical behavior when using the constraint not(selected(join(' ',/r/path/to/repeatgroup[position[./group/repeatfield and position(.) != position(current()/../..)]/group/repeatfield),.)) inside nested repeats. In theory, the form works fine but as long as I select or insert the same values exactly in the same order within subsequent repeats. Let's say I have two nested repeat groups: RG1 and RG2. In the first loop of RG1 I select the RG2 classes 'a', 'b', 'c'. In the second loop, I can still select the classes 'a', 'b', 'c' but only in the same order as the first loop, otherwise the form says the answer is not allowed (e.g. if I select 'b', 'c', 'a'). I understand that it is logical from the constraint's point of view, but for my survey, I have to insert the classes the way they appear which is not necessarily the same order as the time before. Any ideas? Thank you very much.

2. What app or server are you using and on what device and operating system? Include version numbers.
I am using XLSForm to design the form and Central on DigitalOcean to test it
3. What you have you tried to fix the problem?
I have searched for solutions but I haven't found yet any topic specifically to this problem related to nested repeat groups.
4. What steps can we take to reproduce the problem?
I would try the attached xls form by selecting the choices 'a', 'b', 'c' in the first loop and the same choices but in a different order ('b', 'c', 'a') the second loop.
5. Anything else we should know or have? If you have a test form or screenshots or logs, attach below.nested-repeat-selects.xlsx (6.7 KB)