only check for duplicates for let variables

lib/6to5/traverse/scope.js

@@ -169,7 +169,7 @@ var programReferenceVisitor = {
 var blockVariableVisitor = {
   enter: function (node, parent, scope, context, add) {
     if (t.isBlockScoped(node)) {
-      add(node, false, true);
+      add(node, false, t.isLet(node));
     } else if (t.isScope(node)) {
       context.stop();
     }