There was something that I had noticed these last few months with dual sync. Leaving aside the fact that I think it seems to take longer and longer when you swap skills with the same person, is that you can swap the same skill more than once.
The first time I noticed this was thanks to Elena. The first time we exchanged was her affinity for darkness, and on my part the affinity for lightning. Two months later, one morning I noticed that Elena had the plant affinity ability, which was the first one I had exchanged with Alice. When I looked at my status, I noticed that the affinity to darkness had advanced to rank A.
I don't know about other skills, but I more or less understand what is required to bring the affinities at least up to rank (S).
I was sure I hadn't done enough to move that affinity up, and that's when I noticed that, when this skill swap is done, if I already have the skill, it strengthens it.