This repo is a small reproduction of a bug I stumbled upon.
It has generic MediatR notification handlers that get triggered twice when one of the generics has no specialised handler registered.
There is a Github Issue from another user how has the same problem: jbogard/MediatR#884
It turns out that this is not a MediatR issue and instead a bug in the Microsoft DI container: dotnet/runtime#87017 (comment)