diff --git a/klippy/reactor.py b/klippy/reactor.py
index 412d53ed..f9bedcf3 100644
--- a/klippy/reactor.py
+++ b/klippy/reactor.py
@@ -1,6 +1,6 @@
 # File descriptor and timer event helper
 #
-# Copyright (C) 2016-2020  Kevin O'Connor <kevin@koconnor.net>
+# Copyright (C) 2016-2025  Kevin O'Connor <kevin@koconnor.net>
 #
 # This file may be distributed under the terms of the GNU GPLv3 license.
 import os, gc, select, math, time, logging, queue
@@ -14,6 +14,7 @@ class ReactorTimer:
     def __init__(self, callback, waketime):
         self.callback = callback
         self.waketime = waketime
+        self.timer_is_running = False
 
 class ReactorCompletion:
     class sentinel: pass
@@ -118,6 +119,8 @@ class SelectReactor:
         return tuple(self._last_gc_times)
     # Timers
     def update_timer(self, timer_handler, waketime):
+        if timer_handler.timer_is_running:
+            return
         timer_handler.waketime = waketime
         self._next_timer = min(self._next_timer, waketime)
     def register_timer(self, callback, waketime=NEVER):
@@ -155,7 +158,9 @@ class SelectReactor:
             waketime = t.waketime
             if eventtime >= waketime:
                 t.waketime = self.NEVER
+                t.timer_is_running = True
                 t.waketime = waketime = t.callback(eventtime)
+                t.timer_is_running = False
                 if g_dispatch is not self._g_dispatch:
                     self._next_timer = min(self._next_timer, waketime)
                     self._end_greenlet(g_dispatch)