!str #16069 disallows using one Keyed sink/source ambiguously (many times)

This can / should wait for Patrik's move-all-the-things PR.
Refs #16069 
The idea was very witty, but sadly can't go down that road - the asymmetry caused by such equals override is really crazy (and `contains()` wouldnt work anyway). Staying with `forall match eq`.