diff --git a/src/trigger.rs b/src/trigger.rs
index b62998c..c968cbc 100644
--- a/src/trigger.rs
+++ b/src/trigger.rs
@@ -8,7 +8,7 @@ use std::{
 use pin_project_lite::pin_project;
 use slab::Slab;
 
-type WakerList = Arc<Mutex<Slab<Waker>>>;
+type WakerList = Arc<Mutex<Slab<Option<Waker>>>>;
 type TriggerState = Arc<AtomicBool>;
 
 #[derive(Debug, Clone)]
@@ -34,7 +34,7 @@ impl Subscriber {
             return SubscriberState::Triggered;
         }
 
-        let waker = cx.waker().clone();
+        let waker = Some(cx.waker().clone());
 
         SubscriberState::Waiting(if let Some(key) = key {
             tracing::trace!("trigger::Subscriber: updating waker for key: {}", key);
@@ -125,8 +125,8 @@ impl Future for Receiver {
 
 #[derive(Debug, Clone)]
 pub struct Sender {
-    wakers: WakerList,
     state: TriggerState,
+    wakers: WakerList,
 }
 
 impl Sender {
@@ -135,11 +135,24 @@ impl Sender {
     }
 
     pub fn trigger(&self) {
-        let wakers = self.wakers.lock().unwrap();
-        self.state.store(true, std::sync::atomic::Ordering::SeqCst);
-        for (key, waker) in wakers.iter() {
-            tracing::trace!("trigger::Sender: wake up waker with key: {}", key);
-            waker.wake_by_ref();
+        if self.state.swap(true, std::sync::atomic::Ordering::SeqCst) {
+            return;
+        }
+
+        let mut wakers = self.wakers.lock().unwrap();
+        for (key, waker) in wakers.iter_mut() {
+            match waker.take() {
+                Some(waker) => {
+                    tracing::trace!("trigger::Sender: wake up waker with key: {}", key);
+                    waker.wake();
+                }
+                None => {
+                    tracing::trace!(
+                        "trigger::Sender: nop: waker already triggered with key: {}",
+                        key
+                    );
+                }
+            }
         }
     }
 }