I think, I have an idea, how this happened. Due to some inacurracy in my client code, I sent two fetch-calls successively. This resulted in two invocations of the ratings api from two threads in parallel :-/

I guess, a simple synchronized modificator could avoid this!?

Thanks for the investigation! That's indeed something I had not in mind when implementing it: the possibility for two threads to call this in parallel. So indeed it's more likely related to that.