gh-158208: Don't write a duplicate xmlns attribute in xml.dom.minidom

Lib/test/test_minidom.py

@@ -702,6 +702,28 @@ def testWriteXMLNotANamespaceDeclaration(self):
                 '<child xmlns="" xmlnsabc="v"/></root>')
         dom.unlink()
 
+    def testWriteXMLOwnDefaultNamespaceDeclaration(self):
+        # An element in no namespace which declares the default namespace
+        # itself is written with that declaration only.
+        dom = Document()
+        svg = dom.appendChild(dom.createElement("svg"))
+        svg.setAttribute("xmlns", "http://www.w3.org/2000/svg")
+        svg.appendChild(dom.createElement("g"))
+        self.assertEqual(dom.documentElement.toxml(),
+                '<svg xmlns="http://www.w3.org/2000/svg"><g/></svg>')
+        dom.unlink()
+
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "root"))
+        svg = root.appendChild(dom.createElement("svg"))
+        svg.setAttribute("xmlns", "http://www.w3.org/2000/svg")
+        svg.appendChild(dom.createElement("g"))
+        self.assertEqual(dom.documentElement.toxml(),
+                '<root xmlns="http://xml.python.org/ns">'
+                '<svg xmlns="http://www.w3.org/2000/svg"><g/></svg></root>')
+        dom.unlink()
+
     def testWriteXMLDoesNotModifyDocument(self):
         dom = Document()
         root = dom.appendChild(

Lib/xml/dom/minidom.py

@@ -449,7 +449,8 @@ def _fixup_namespaces(element, nsmap):
     elif nsmap.get(None) and ':' not in element.tagName:
         # The element is in no namespace, undeclare the default one.
         nsmap = _bind_namespace(nsmap, inherited, None, None)
-        declarations.append(("xmlns", ""))
+        if not element.hasAttribute("xmlns"):
+            declarations.append(("xmlns", ""))
 
     items = []
     prefixes = None  # namespace URI -> prefix, built only when needed

Misc/NEWS.d/next/Library/2026-11-03-17-53-55.gh-issue-158208.ZOQnym.rst

@@ -0,0 +1,2 @@
+Fix :mod:`xml.dom.minidom` writing a duplicate ``xmlns`` attribute for an
+element in no namespace that has its own ``xmlns`` attribute.