diff --git a/lib/6to5/file.js b/lib/6to5/file.js
index 479e233ee6..be2f61a613 100644
--- a/lib/6to5/file.js
+++ b/lib/6to5/file.js
@@ -10,11 +10,12 @@ var t         = require("./types");
 var _         = require("lodash");
 
 function File(opts) {
-  this.dynamicImports = [];
-  this.opts           = File.normaliseOptions(opts);
-  this.transformers   = this.getTransformers();
-  this.uids           = {};
-  this.ast            = {};
+  this.dynamicImports   = [];
+  this.dynamicImportIds = {};
+  this.opts             = File.normaliseOptions(opts);
+  this.transformers     = this.getTransformers();
+  this.uids             = {};
+  this.ast              = {};
 }
 
 File.helpers = [
@@ -164,11 +165,20 @@ File.prototype.parseShebang = function (code) {
   return code;
 };
 
-File.prototype.addImport = function (id, source) {
-  var specifiers = [t.importSpecifier(t.identifier("default"), id)];
-  var declar = t.importDeclaration(specifiers, t.literal(source));
-  declar._blockHoist = 3;
-  this.dynamicImports.push(declar);
+File.prototype.addImport = function (source, name) {
+  name = name || source;
+  var id = this.dynamicImportIds[name];
+
+  if (!id) {
+    id = this.dynamicImportIds[name] = this.generateUidIdentifier(name);
+
+    var specifiers = [t.importSpecifier(t.identifier("default"), id)];
+    var declar = t.importDeclaration(specifiers, t.literal(source));
+    declar._blockHoist = 3;
+    this.dynamicImports.push(declar);
+  }
+
+  return id;
 };
 
 File.prototype.addHelper = function (name) {
@@ -277,7 +287,7 @@ File.prototype.generate = function () {
 };
 
 File.prototype.generateUid = function (name, scope) {
-  name = t.toIdentifier(name);
+  name = t.toIdentifier(name).replace(/^_+/, "");
 
   scope = scope || this.scope;
 
diff --git a/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js b/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js
index beb19dad7a..cda714330c 100644
--- a/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js
+++ b/lib/6to5/transformation/transformers/optional-bluebird-coroutines.js
@@ -38,8 +38,6 @@ exports._Function = function (node, callId) {
 exports.Function = function (node, parent, file) {
   if (!node.async || node.generator) return;
 
-  var id = t.identifier("Bluebird");
-  file.addImport(id, "bluebird");
-
+  var id = file.addImport("bluebird");
   return exports._Function(node, t.memberExpression(id, t.identifier("coroutine")));
 };
diff --git a/lib/6to5/transformation/transformers/optional-core-aliasing.js b/lib/6to5/transformation/transformers/optional-core-aliasing.js
index bd2fb2f572..a67bf045ec 100644
--- a/lib/6to5/transformation/transformers/optional-core-aliasing.js
+++ b/lib/6to5/transformation/transformers/optional-core-aliasing.js
@@ -12,8 +12,7 @@ exports.optional = true;
 
 exports.ast = {
   enter: function (ast, file) {
-    file._coreId = file.generateUidIdentifier("core");
-    file.addImport(file._coreId, "core-js/library");
+    file._coreId = file.addImport("core-js/library", "core");
   },
 
   exit: function (ast, file) {