diff --git a/lib/6to5/transformation/transformers/es6-let-scoping.js b/lib/6to5/transformation/transformers/es6-let-scoping.js
index d6ccf9078f..65f8cf9d9c 100644
--- a/lib/6to5/transformation/transformers/es6-let-scoping.js
+++ b/lib/6to5/transformation/transformers/es6-let-scoping.js
@@ -162,21 +162,32 @@ LetScoping.prototype.noClosure = function () {
  */
 
 LetScoping.prototype.remap = function () {
-  var replacements = this.info.duplicates;
-  var block        = this.block;
+  var duplicates = this.info.duplicates;
+  var block      = this.block;
+  var scope      = this.scope;
 
   if (!this.info.hasDuplicates) return;
 
-  var replace = function (node, parent, scope, context, replacements) {
+  var replace = function (node, parent, scope, context, duplicates) {
     if (!t.isIdentifier(node)) return;
     if (!t.isReferenced(node, parent)) return;
-    if (scope && scope.hasOwn(node.name)) return;
-    node.name = replacements[node.name] || node.name;
+
+    var duplicate = duplicates[node.name];
+    if (!duplicate) return;
+
+    var own = scope.get(node.name, true);
+    if (own === duplicate.node) {
+      node.name = duplicate.uid;
+    } else {
+      // scope already has it's own declaration that doesn't
+      // match the one we have a stored replacement for
+      context.skip();
+    }
   };
 
   var traverseReplace = function (node, parent) {
     replace(node, parent);
-    traverse(node, { enter: replace }, null, replacements);
+    traverse(node, { enter: replace }, scope, duplicates);
   };
 
   var loopParent = this.loopParent;
@@ -186,7 +197,7 @@ LetScoping.prototype.remap = function () {
     traverseReplace(loopParent.update, loopParent);
   }
 
-  traverse(block, { enter: replace }, null, replacements);
+  traverse(block, { enter: replace }, scope, duplicates);
 };
 
 /**
@@ -221,12 +232,14 @@ LetScoping.prototype.getInfo = function () {
   };
 
   var duplicates = function (id, key) {
-    var has = scope.parentGet(key);
-
-    if (has && has !== id) {
-      // there's a variable with this exact name in an upper scope so we need
-      // to generate a new name
-      opts.duplicates[key] = id.name = file.generateUidIdentifier(key, scope).name;
+    // there's a variable with this exact name in an upper scope so we need
+    // to generate a new name
+    if (scope.parentHas(key, true)) {
+      var duplicate = opts.duplicates[key] = {
+        uid: file.generateUidIdentifier(key, scope).name,
+        node: id
+      };
+      id.name = duplicate.uid;
       opts.hasDuplicates = true;
     }
   };
@@ -305,7 +318,7 @@ LetScoping.prototype.checkLoop = function () {
 
       if (replace) return t.inherits(replace, node);
     }
-  }, null, has);
+  }, this.scope, has);
 
   return has;
 };
@@ -332,7 +345,7 @@ LetScoping.prototype.hoistVarDeclarations = function () {
         return context.skip();
       }
     }
-  }, null, this);
+  }, this.scope, this);
 };
 
 /**
@@ -347,7 +360,8 @@ LetScoping.prototype.getParams = function (params) {
   var info = this.info;
   params = _.cloneDeep(params);
   _.each(params, function (param) {
-    param.name = info.duplicates[param.name] || param.name;
+    var duplicate = info.duplicates[param.name];
+    if (duplicate) param.name = duplicate.uid;
   });
   return params;
 };
@@ -388,12 +402,12 @@ LetScoping.prototype.getLetReferences = function () {
             // push this badboy
             state.self.letReferences[node.name] = node;
           }
-        }, null, state);
+        }, scope, state);
 
         return context.skip();
       }
     }
-  }, null, state);
+  }, this.scope, state);
 
   return state.closurify;
 };
diff --git a/test/fixtures/transformation/es7-abstract-references/private/expected.js b/test/fixtures/transformation/es7-abstract-references/private/expected.js
index 1af25539ca..d79155a923 100644
--- a/test/fixtures/transformation/es7-abstract-references/private/expected.js
+++ b/test/fixtures/transformation/es7-abstract-references/private/expected.js
@@ -4,19 +4,19 @@ var A = new WeakMap();
 var B = new WeakMap(),
     C = new WeakMap();
 var D = (function () {
-  var F = new WeakMap(),
-      G = new WeakMap();
-  var E = new WeakMap();
+  var _F = new WeakMap(),
+      _G = new WeakMap();
+  var _E = new WeakMap();
   function D() {}
 
   return D;
 })();
 
 var H = (function () {
-  var J = new WeakMap(),
-      K = new WeakMap();
-  var I = new WeakMap();
+  var _J = new WeakMap(),
+      _K = new WeakMap();
+  var _I = new WeakMap();
   function H() {}
 
   return H;
-})();
+})();
\ No newline at end of file
diff --git a/test/fixtures/transformation/es7-array-comprehension/array-expression-single-if/expected.js b/test/fixtures/transformation/es7-array-comprehension/array-expression-single-if/expected.js
index 37496b18dc..0f3f625138 100644
--- a/test/fixtures/transformation/es7-array-comprehension/array-expression-single-if/expected.js
+++ b/test/fixtures/transformation/es7-array-comprehension/array-expression-single-if/expected.js
@@ -4,9 +4,9 @@ var arr = (function () {
   var _arr = [];
 
   for (var _iterator = [1, 2, 3][Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
-    var _i = _step.value;
-    if (_i > 1) {
-      _arr.push(_i * _i);
+    var i = _step.value;
+    if (i > 1) {
+      _arr.push(i * i);
     }
   }
 
diff --git a/test/fixtures/transformation/es7-array-comprehension/multiple-if/expected.js b/test/fixtures/transformation/es7-array-comprehension/multiple-if/expected.js
index c5ba3b5331..6b486b45f5 100644
--- a/test/fixtures/transformation/es7-array-comprehension/multiple-if/expected.js
+++ b/test/fixtures/transformation/es7-array-comprehension/multiple-if/expected.js
@@ -6,9 +6,9 @@ var seattlers = (function () {
   for (var _iterator = countries[Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
     var customers = _step.value;
     for (var _iterator2 = customers[Symbol.iterator](), _step2; !(_step2 = _iterator2.next()).done;) {
-      var _c = _step2.value;
-      if (_c.city == "Seattle") {
-        _seattlers.push({ name: _c.name, age: _c.age });
+      var c = _step2.value;
+      if (c.city == "Seattle") {
+        _seattlers.push({ name: c.name, age: c.age });
       }
     }
   }
diff --git a/test/fixtures/transformation/es7-array-comprehension/single-if/expected.js b/test/fixtures/transformation/es7-array-comprehension/single-if/expected.js
index 25915fc7f0..6e95f9cadc 100644
--- a/test/fixtures/transformation/es7-array-comprehension/single-if/expected.js
+++ b/test/fixtures/transformation/es7-array-comprehension/single-if/expected.js
@@ -4,9 +4,9 @@ var arr = (function () {
   var _arr = [];
 
   for (var _iterator = nums[Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
-    var _i = _step.value;
-    if (_i > 1) {
-      _arr.push(_i * _i);
+    var i = _step.value;
+    if (i > 1) {
+      _arr.push(i * i);
     }
   }
 
diff --git a/test/fixtures/transformation/es7-exponentian-operator/comprehensive/exec.js b/test/fixtures/transformation/es7-exponentian-operator/comprehensive/exec.js
new file mode 100644
index 0000000000..5a7ac52277
--- /dev/null
+++ b/test/fixtures/transformation/es7-exponentian-operator/comprehensive/exec.js
@@ -0,0 +1,21 @@
+assert.equal(8, 2 ** 3);
+assert.equal(24, 3 * 2 ** 3);
+var x = 2;
+assert.equal(8, 2 ** ++x);
+assert.equal(1, (2 ** -1) * 2);
+
+var calls = 0;
+var q = {q: 3};
+var o = {
+  get p() {
+    calls++;
+    return q;
+  }
+};
+
+o.p.q **= 2;
+assert.equal(1, calls);
+assert.equal(9, o.p.q);
+
+assert.equal(512, 2 ** (3 ** 2));
+assert.equal(512, 2 ** 3 ** 2);