Multiple namespace loading behavior

repl

user=> (require 'test-ns)
Loading...
nil
user=> (require 'test-ns)
nil
user=> (require 'test_ns)
"Loading...
nil
user=> (require 'test_ns)
nil

test_ns.clj

(ns test-ns)

(println "Loading...)

Weird. Note that doing the require of test-ns first causes only test-ns to be added to the clojure.core/*loaded-libs* set, whereas doing require of test_ns first causes both test-ns and test_ns to be added to clojure.core/*loaded-libs*.

Addendum: There is additional error checking in require that checks whether the namespace with the require'd name was created when require is nearly done, but it is only enabled if you use the :as clause. Example given the contents of test_ns.clj above:

user=> (require '[test_ns :as myalias])
Loading...
Syntax error compiling at (REPL:1:1).
namespace 'test_ns' not found after loading '/test_ns'

That additional error checking is also enabled by doing (use 'test_ns)