gcode: Raise an error if a duplicate command is registered

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

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:

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']: