Interesting, I hadn't thought about it since, for my usage, setting up keys is pretty much always a one-off event (and pretty rare on top of that). What are the consequences of dropping your key into authorized_keys files multiple times? Tried it out on a Debian box, and didn't see any issues other than the obvious clutter.

"looks bad" :-)

But it's a bad habit to get into. Or better yet, it's better to think in idempotent operations when doing systems work.