Repeated applications of `ns` form at the REPL are not additive

user=> (ns my-test-ns.core (:require [clojure.string :as string]))
nil
my-test-ns.core=> (def a string/blank?)
#'my-test-ns.core/a
my-test-ns.core=> (ns my-test-ns.core)
nil
my-test-ns.core=> (def a string/blank?)
#'my-test-ns.core/a
my-test-ns.core=>

ClojureScript:cljs.user> (ns my-test-ns.core (:require [clojure.string :as string]))
true
ClojureScript:my-test-ns.core> (def a string/blank?)
#<function clojure$string$blank_QMARK_(s){
return goog.string.isEmptySafe(s);
}>
ClojureScript:my-test-ns.core> (ns my-test-ns.core)
nil
ClojureScript:my-test-ns.core> (def a string/blank?)
WARNING: No such namespace: string, could not locate string.cljs at line 1 <cljs repl>
WARNING: Use of undeclared Var string/blank? at line 1 <cljs repl>
repl:13
throw e__3919__auto__;
      ^
ReferenceError: string is not defined
    at repl:1:109
    at repl:9:3
    at repl:14:4
    at Object.exports.runInThisContext (vm.js:74:17)
    at Domain.<anonymous> ([stdin]:41:34)
    at Domain.run (domain.js:197:16)
    at Socket.<anonymous> ([stdin]:40:25)
    at Socket.emit (events.js:107:17)
    at readableAddChunk (_stream_readable.js:163:16)
    at Socket.Readable.push (_stream_readable.js:126:10)
ClojureScript:my-test-ns.core>