!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 
I realized that my answer wasn't that clear. We should not check `checkAmbigiousKeyedElement` in `checkAddOrReplaceSourceSinkPrecondition`, since there will be collisions. The node should be in the graph already.