diff --git a/klippy/extras/homing_override.py b/klippy/extras/homing_override.py
index 62a97aa4..f69298aa 100644
--- a/klippy/extras/homing_override.py
+++ b/klippy/extras/homing_override.py
@@ -12,6 +12,7 @@ class HomingOverride:
         self.script = config.get('gcode')
         self.in_script = False
         self.gcode = self.printer.lookup_object('gcode')
+        self.gcode.register_command("G28", None)
         self.gcode.register_command("G28", self.cmd_G28)
     def cmd_G28(self, params):
         if self.in_script:
diff --git a/klippy/extras/virtual_sdcard.py b/klippy/extras/virtual_sdcard.py
index 5a3a09a2..5d2780d7 100644
--- a/klippy/extras/virtual_sdcard.py
+++ b/klippy/extras/virtual_sdcard.py
@@ -19,6 +19,7 @@ class VirtualSD:
         self.work_timer = None
         # Register commands
         self.gcode = printer.lookup_object('gcode')
+        self.gcode.register_command('M21', None)
         for cmd in ['M20', 'M21', 'M23', 'M24', 'M25', 'M26', 'M27']:
             self.gcode.register_command(cmd, getattr(self, 'cmd_' + cmd))
         for cmd in ['M28', 'M29', 'M30']:
diff --git a/klippy/gcode.py b/klippy/gcode.py
index d381620d..6cee4570 100644
--- a/klippy/gcode.py
+++ b/klippy/gcode.py
@@ -62,6 +62,8 @@ class GCodeParser:
             if cmd in self.base_gcode_handlers:
                 del self.base_gcode_handlers[cmd]
             return
+        if cmd in self.ready_gcode_handlers:
+            raise error("gcode command %s already registered" % (cmd,))
         if not (len(cmd) >= 2 and not cmd[0].isupper() and cmd[1].isdigit()):
             origfunc = func
             func = lambda params: origfunc(self.get_extended_params(params))