Since Clojure 1.2 defmulti behaves similarly to defonce. This fact, and how one deals with it when doing interactive development, does not seem to be documented anywhere. This issue pops up fairly regularly in the #clojure channel.
Commit that introduced the change: https://github.com/clojure/clojure/commit/1b8d5001ba094053b24c55829994785be422cfbf
I can volunteer to update the docstrings and (if I get access to it) the clojure.org page.