Consider two IO streams with different frequency; let's say IO stream 1 has nc at 3-hourly and IO stream 2 has qc at 6-hourly. The user sets these two IO streams with the same name of "clouds"

All well and good until a restart, one of the rhist files will overwrite the other, introducing a runtime error upon restart of a missing variable (either nc or qc).

In summary, same filename for "same output but different freq" --> same rhists --> overwriting --> bug.

Arguably, the user can sidestep this pretty easily, but we should harden our IO logic to avoid it altogether.

Discovered accidentally (and took me a good half hour to realize it) while setting up E3SM-Project/E3SM#6580