diff --git a/crates/obscura-dom/src/tree.rs b/crates/obscura-dom/src/tree.rs
index eeef499..a00f233 100644
--- a/crates/obscura-dom/src/tree.rs
+++ b/crates/obscura-dom/src/tree.rs
@@ -136,7 +136,11 @@ impl Node {
 
     pub fn set_attribute(&mut self, name: &str, value: String) {
         if let NodeData::Element { attrs, .. } = &mut self.data {
-            if let Some(attr) = attrs.iter_mut().find(|a| a.name.local.as_ref() == name) {
+            // Match by qualified name, consistent with get_attribute and the
+            // remove_attribute op. A parsed namespaced attribute is stored with
+            // a separate prefix (e.g. xlink:href -> prefix="xlink", local="href");
+            // matching on local name alone would miss it and push a duplicate.
+            if let Some(attr) = attrs.iter_mut().find(|a| a.qualified_name_eq(name)) {
                 attr.value = value;
             } else {
                 attrs.push(Attribute {
diff --git a/crates/obscura-js/src/runtime.rs b/crates/obscura-js/src/runtime.rs
index 03a12fc..d457e91 100644
--- a/crates/obscura-js/src/runtime.rs
+++ b/crates/obscura-js/src/runtime.rs
@@ -1662,6 +1662,22 @@ mod tests {
         assert_eq!(v, serde_json::json!("#icon|#icon|id,xlink:href"));
     }
 
+    #[test]
+    fn set_attribute_updates_a_parsed_namespaced_attribute_in_place() {
+        // setAttribute matched the stored attribute by local name only, so a
+        // parsed `xlink:href` (prefix=xlink, local=href) was never found by the
+        // qualified name "xlink:href": the update was pushed as a *second*
+        // attribute, getAttribute kept returning the stale original, and the
+        // element serialized `xlink:href` twice.
+        let mut rt = setup_runtime(
+            r##"<html><body><svg><use id="u" xlink:href="#a"></use></svg></body></html>"##,
+        );
+        let v = rt
+            .evaluate("(function(){var u=document.getElementById('u');u.setAttribute('xlink:href','#b');var dup=(u.outerHTML.match(/xlink:href/g)||[]).length;return u.getAttribute('xlink:href')+'|'+u.getAttributeNS('http://www.w3.org/1999/xlink','href')+'|'+u.getAttributeNames().join(',')+'|'+dup;})()")
+            .unwrap();
+        assert_eq!(v, serde_json::json!("#b|#b|id,xlink:href|1"));
+    }
+
     #[test]
     fn set_attribute_ns_validates_namespace_constraints() {
         let mut rt = setup_runtime("<html><body></body></html>");