diff --git a/dom/bindings/parser/WebIDL.py b/dom/bindings/parser/WebIDL.py
index 6d548960cb18..863ceb91fca2 100644
--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -1101,6 +1101,13 @@ class IDLInterfaceOrNamespace(IDLInterfaceOrInterfaceMixinOrNamespace):
 
         for mixin in sorted(self.includedMixins,
                             key=lambda x: x.identifier.name):
+            for mixinMember in mixin.members:
+                for member in self.members:
+                    if mixinMember.identifier.name == member.identifier.name:
+                        raise WebIDLError(
+                            "Multiple definitions of %s on %s coming from 'includes' statements" %
+                            (member.identifier.name, self),
+                            [mixinMember.location, member.location])
             self.members.extend(mixin.members)
 
         for ancestor in self.getInheritedInterfaces():
diff --git a/dom/bindings/parser/tests/test_interfacemixin.py b/dom/bindings/parser/tests/test_interfacemixin.py
index a97400b3b632..ac36b51ea0ed 100644
--- a/dom/bindings/parser/tests/test_interfacemixin.py
+++ b/dom/bindings/parser/tests/test_interfacemixin.py
@@ -337,6 +337,44 @@ def WebIDLTest(parser, harness):
     harness.ok(threw,
                "Should fail if an interface mixin includes maplike")
 
+    parser = parser.reset()
+    threw = False
+    try:
+        parser.parse("""
+            interface Interface {
+                attribute short attr;
+            };
+            interface mixin Mixin {
+                attribute short attr;
+            };
+            Interface includes Mixin;
+        """)
+        results = parser.finish()
+    except:
+        threw = True
+    harness.ok(threw,
+               "Should fail if the included mixin interface has duplicated member")
+
+    parser = parser.reset()
+    threw = False
+    try:
+        parser.parse("""
+            interface Interface {};
+            interface mixin Mixin1 {
+                attribute short attr;
+            };
+            interface mixin Mixin2 {
+                attribute short attr;
+            };
+            Interface includes Mixin1;
+            Interface includes Mixin2;
+        """)
+        results = parser.finish()
+    except:
+        threw = True
+    harness.ok(threw,
+               "Should fail if the included mixin interfaces have duplicated member")
+
     parser = parser.reset()
     parser.parse("""
         [Global] interface Window {};