Use atomic swap to ensure only 1 wake operation procedes

src/lib.rs

@@ -207,7 +207,7 @@ struct SafeWaker {
 impl Wake for SafeWaker {
     fn wake(self: std::sync::Arc<Self>) {
         if let Some(task) = Weak::upgrade(&self.task) {
-            if !task.woken.load(Ordering::Acquire) {
+            if !task.woken.swap(true, Ordering::AcqRel) {
                 if let Some(runtime) = Weak::upgrade(&self.runtime) {
                     let mut inner = runtime.lock().unwrap();
                     inner.wake(task);