cs & thread: avoid accumulating post-atomic callbacks

Only one instance of each callback is needed. Allowing them to pile up
is inefficient, and possibly it can trigger a reaction that causes
even more to pile up.

racket/src/thread/atomic.rkt

@@ -105,7 +105,14 @@
 ;; no race with the scheduler
 (define (add-end-atomic-callback! cb)
   (host:disable-interrupts)
-  (end-atomic-callback (cons cb (end-atomic-callback)))
+  (define all-cbs (end-atomic-callback))
+  (let loop ([cbs all-cbs])
+    (cond
+      [(eq? cbs 0)
+       (end-atomic-callback (cons cb all-cbs))]
+      [else
+       (unless (eq? (car cbs) cb)
+         (loop (cdr cbs)))]))
   (host:enable-interrupts))
 
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