diff --git a/lib/utils/mixin.html b/lib/utils/mixin.html
index 91dbdbfe..85a04b0e 100644
--- a/lib/utils/mixin.html
+++ b/lib/utils/mixin.html
@@ -51,9 +51,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
   Polymer.dedupingMixin = function(mixin) {
     mixin = cachingMixin(mixin);
     // maintain a unique id for each mixin
-    if (!mixin.__id) {
-      mixin.__dedupeId = ++dedupeId;
-    }
+    mixin.__dedupeId = ++dedupeId;
     return function(base) {
       let baseSet = base.__mixinSet;
       if (baseSet && baseSet[mixin.__dedupeId]) {