!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 
Thanks @bantonsson, makes sense not to build that collection - updating

@patriknw sadly it doesn't work just with `contains`, because we need to compare the things inside the vertices - the undefined sink/source are lucky because they are the vertex, and they are what we need to compare here on the other hand the unwrapping is necessary.