gh-152486: Fix duplicated zip64 extra in local file entry

Lib/test/test_zipfile/test_core.py

@@ -1189,6 +1189,34 @@ def test_generated_valid_zip64_extra(self):
                     self.assertEqual(zinfo.header_offset, expected_header_offset)
                     self.assertEqual(zf.read(zinfo), expected_content)
 
+    def test_generated_valid_zip64_extra_in_local_entry(self):
+        """Should not write duplicated zip64 fields to the local file entry."""
+        fh = io.BytesIO()
+        with zipfile.ZipFile(fh, 'w') as zh:
+            zinfo = zipfile.ZipInfo('strfile')
+            zinfo.extra = (
+                # zip64 (should be stripped)
+                b'\x01\x00\x10\x00'
+                b'\x00\x00\x00\x00\x00\x00\x00\x00'
+                b'\x00\x00\x00\x00\x00\x00\x00\x00'
+
+                # invalid tail
+                b'zzz'
+            )
+            zh.writestr(zinfo, self.data)
+
+        fh.seek(0)
+        fh.seek(zinfo.header_offset)
+        entry = fh.read(zh.start_dir - zinfo.header_offset - zinfo.compress_size)
+        header = struct.unpack_from(zipfile.structFileHeader, entry)
+        extra_bytes = entry[-header[zipfile._FH_EXTRA_FIELD_LENGTH]:]
+        self.assertEqual(extra_bytes, (
+            b'\x01\x00\x10\x00'
+            b'!e\x00\x00\x00\x00\x00\x00'
+            b'!e\x00\x00\x00\x00\x00\x00'
+            b'zzz'
+        ))
+
     def test_force_zip64(self):
         """Test that forcing zip64 extensions correctly notes this in the zip file"""
 

Lib/zipfile/__init__.py

@@ -550,8 +550,13 @@ def FileHeader(self, zip64=None):
             zip64 = file_size > ZIP64_LIMIT or compress_size > ZIP64_LIMIT
         if zip64:
             fmt = '<HHQQ'
-            extra = extra + struct.pack(fmt,
-                                        1, struct.calcsize(fmt)-4, file_size, compress_size)
+
+            # Prepend a ZIP64 field to extra data
+            extra = struct.pack(
+                fmt, 1, struct.calcsize(fmt)-4,
+                file_size, compress_size
+            ) + _Extra.strip(extra, (1,))
+
             file_size = 0xffffffff
             compress_size = 0xffffffff
             min_version = ZIP64_VERSION

Misc/NEWS.d/next/Core_and_Builtins/2026-06-29-03-43-41.gh-issue-152486.kASYh8.rst

@@ -0,0 +1 @@
+Fix an issue where duplicated Zip64 extra fields were written if :attr:`ZipInfo.extra <zipfile.ZipInfo.extra>` already contained a Zip64 field.