diff --git a/py/makemoduledefs.py b/py/makemoduledefs.py
index 612f3d29a..521245bdc 100644
--- a/py/makemoduledefs.py
+++ b/py/makemoduledefs.py
@@ -59,18 +59,21 @@ def generate_module_table_header(modules):
     """
 
     # Print header file for all external modules.
-    mod_defs = []
+    mod_defs = set()
     print("// Automatically generated by makemoduledefs.py.\n")
     for module_name, obj_module, enabled_define in modules:
         mod_def = "MODULE_DEF_{}".format(module_name.upper())
-        mod_defs.append(mod_def)
+        mod_defs.add(mod_def)
         print(
             (
                 "#if ({enabled_define})\n"
                 "    extern const struct _mp_obj_module_t {obj_module};\n"
+                "    #undef {mod_def}\n"
                 "    #define {mod_def} {{ MP_ROM_QSTR({module_name}), MP_ROM_PTR(&{obj_module}) }},\n"
                 "#else\n"
-                "    #define {mod_def}\n"
+                "    #ifndef {mod_def}\n"
+                "        #define {mod_def}\n"
+                "    #endif\n"
                 "#endif\n"
             ).format(
                 module_name=module_name,
@@ -82,7 +85,7 @@ def generate_module_table_header(modules):
 
     print("\n#define MICROPY_REGISTERED_MODULES \\")
 
-    for mod_def in mod_defs:
+    for mod_def in sorted(mod_defs):
         print("    {mod_def} \\".format(mod_def=mod_def))
 
     print("// MICROPY_REGISTERED_MODULES")