gh-106560: Fix redundant declarations in Python/frozen.c (#112612)

Avoid duplicated declarations of "extern" functions in
Python/frozen.c.

Compiler warnings seen by building Python with gcc -Wredundant-decls.

Python/frozen.c

@@ -80,21 +80,15 @@ extern PyObject *_Py_get__sitebuiltins_toplevel(void);
 extern PyObject *_Py_get_genericpath_toplevel(void);
 extern PyObject *_Py_get_ntpath_toplevel(void);
 extern PyObject *_Py_get_posixpath_toplevel(void);
-extern PyObject *_Py_get_posixpath_toplevel(void);
 extern PyObject *_Py_get_os_toplevel(void);
 extern PyObject *_Py_get_site_toplevel(void);
 extern PyObject *_Py_get_stat_toplevel(void);
 extern PyObject *_Py_get_importlib_util_toplevel(void);
 extern PyObject *_Py_get_importlib_machinery_toplevel(void);
 extern PyObject *_Py_get_runpy_toplevel(void);
 extern PyObject *_Py_get___hello___toplevel(void);
-extern PyObject *_Py_get___hello___toplevel(void);
-extern PyObject *_Py_get___hello___toplevel(void);
-extern PyObject *_Py_get___hello___toplevel(void);
-extern PyObject *_Py_get___phello___toplevel(void);
 extern PyObject *_Py_get___phello___toplevel(void);
 extern PyObject *_Py_get___phello___ham_toplevel(void);
-extern PyObject *_Py_get___phello___ham_toplevel(void);
 extern PyObject *_Py_get___phello___ham_eggs_toplevel(void);
 extern PyObject *_Py_get___phello___spam_toplevel(void);
 extern PyObject *_Py_get_frozen_only_toplevel(void);

Tools/build/freeze_modules.py

@@ -468,6 +468,17 @@ def replace_block(lines, start_marker, end_marker, replacements, file):
     return lines[:start_pos + 1] + replacements + lines[end_pos:]
 
 
+class UniqueList(list):
+    def __init__(self):
+        self._seen = set()
+
+    def append(self, item):
+        if item in self._seen:
+            return
+        super().append(item)
+        self._seen.add(item)
+
+
 def regen_frozen(modules):
     headerlines = []
     parentdir = os.path.dirname(FROZEN_FILE)
@@ -477,7 +488,7 @@ def regen_frozen(modules):
         header = relpath_for_posix_display(src.frozenfile, parentdir)
         headerlines.append(f'#include "{header}"')
 
-    externlines = []
+    externlines = UniqueList()
     bootstraplines = []
     stdliblines = []
     testlines = []