with-redefs is global

gistfile1.clj

(defn inc-it [x] (+ x 1))

(let [hand-off-queue (java.util.concurrent.SynchronousQueue.)
      f1 (future-call
            (fn []
              (with-redefs [inc-it (fn [x] (- x))]
                (.put hand-off-queue "foo")
                (inc-it 10))))
      f2 (future-call
            (fn []
              (.poll hand-off-queue 10 java.util.concurrent.TimeUnit/SECONDS)
              (inc-it 10)))]
  (println "f1 " @f1) ;; -> -10
  (println "f2 " @f2)) ;; -> -10