Run through the Quick Start and go down through to the Browser REPL portion (https://github.com/clojure/clojurescript/wiki/Quick-Start#browser-repl), but exclude the {{:watch}} option from {{repl.clj}}.
Then further down, where the new symbol is introduced
;; ADDED
(defn foo [a b]
(+ a b))
instead cause some duplicate symbols to be introduced in order to provoke compiler warnings:
`
(def a 1)
(def a 1)
(defn foo [a b]
(+ a b))
(defn foo [a b]
(+ a b))
`
Then evaluate the {{require}} statement in the tutorial and observe that the warnings are emitted twice:
ClojureScript:cljs.user> (require '[hello-world.core :as hello])
WARNING: a at line 11 is being replaced at line 12 /Users/mfikes/Desktop/hello_world/src/hello_world/core.cljs
WARNING: foo at line 14 is being replaced at line 16 /Users/mfikes/Desktop/hello_world/src/hello_world/core.cljs
WARNING: a at line 11 is being replaced at line 12 /Users/mfikes/Desktop/hello_world/src/hello_world/core.cljs
WARNING: foo at line 14 is being replaced at line 16 /Users/mfikes/Desktop/hello_world/src/hello_world/core.cljs
nil
