diff --git a/lib/saxes.js b/lib/saxes.js
index a00e0c53..c7b38f8f 100644
--- a/lib/saxes.js
+++ b/lib/saxes.js
@@ -939,6 +939,7 @@ ${XML_NAMESPACE}.`);
         }
       }
 
+      const seen = new Set();
       // Note: do not apply default ns to attributes:
       //   http://www.w3.org/TR/REC-xml-names/#defaulting
       for (const [name, value] of this.attribList) {
@@ -952,6 +953,12 @@ ${XML_NAMESPACE}.`);
           uri,
         };
 
+        const eqname = `{${uri}}${local}`;
+        if (seen.has(eqname)) {
+          this.fail(`Duplicate attribute: ${eqname}`);
+        }
+        seen.add(eqname);
+
         // if there's any attributes with an undefined namespace,
         // then fail on them now.
         if (prefix && prefix !== "xmlns" && !uri) {
@@ -966,6 +973,9 @@ ${XML_NAMESPACE}.`);
     else {
       for (const [name, value] of this.attribList) {
         const a = { name, value };
+        if (this.tag.attributes[name]) {
+          this.fail(`Duplicate attribute: ${name}.`);
+        }
         this.tag.attributes[name] = value;
         this.emitNode("onattribute", a);
       }
diff --git a/test/duplicate-attribute.js b/test/duplicate-attribute.js
index 4c806cc6..d86caaf2 100644
--- a/test/duplicate-attribute.js
+++ b/test/duplicate-attribute.js
@@ -9,6 +9,7 @@ require(".").test({
       attributes: {},
     }],
     ["attribute", { name: "id", value: "hello" }],
+    ["error", "Duplicate attribute: id.\nLine: 0\nColumn: 28\nChar: >"],
     ["attribute", { name: "id", value: "there" }],
     ["opentag", {
       name: "span",