quoted

I'll just send a patch to change "device already registered"
to a low-priority pr_debug message rather than something that
looks like an error.

register_quiet isn't supposed to suppress legit errors - if it does
that's a bug

It does suppress all the err = "…" messages in register_bcache from super.c;
I don't know how often they come up in practice.  From the read_super call
on, errors get reported by the called functions.