diff --git a/packages/accounts-base/accounts_server.js b/packages/accounts-base/accounts_server.js
index 8b4dc17346..50b8bd0d81 100644
--- a/packages/accounts-base/accounts_server.js
+++ b/packages/accounts-base/accounts_server.js
@@ -103,6 +103,10 @@
     return _.extend(user, extra);
   };
   Meteor.accounts.onCreateUserHook = function (options, extra, user) {
+    // add created at timestamp (and protect passed in user object from
+    // modification)
+    user = _.extend({createdAt: +(new Date)}, user);
+
     var fullUser;
 
     if (onCreateUserHook) {
@@ -118,13 +122,22 @@
     }
 
     _.each(validateNewUserHooks, function (hook) {
-      if (!hook(user))
+      if (!hook(fullUser))
         throw new Meteor.Error(403, "User validation failed");
     });
 
-    // XXX check for existing user with duplicate email or username.
-    // better here than the two places we call it (and immediately
-    // follow with an insert)
+    // check for existing user with duplicate email or username.
+    if (fullUser.username &&
+        Meteor.users.findOne({username: fullUser.username}))
+      throw new Meteor.Error(403, "Username already exists.");
+
+    if (fullUser.emails) {
+      var addresses = _.map(fullUser.emails, function (e) {
+        return e.address; });
+      if (Meteor.users.findOne({'emails.address': {$in: addresses}}))
+        throw new Meteor.Error(403, "Email already exists.");
+    }
+
 
     return fullUser;
   };
diff --git a/packages/accounts-base/accounts_tests.js b/packages/accounts-base/accounts_tests.js
index 13f939fe06..4c4d20f015 100644
--- a/packages/accounts-base/accounts_tests.js
+++ b/packages/accounts-base/accounts_tests.js
@@ -39,3 +39,94 @@ Tinytest.add('accounts - updateOrCreateUser', function (test) {
 
 });
 
+Tinytest.add('accounts - onCreateUserHook username', function (test) {
+  var userIn = {
+    username: Meteor.uuid()
+  };
+
+  // user does not already exist. return a user object with fields set.
+  var userOut = Meteor.accounts.onCreateUserHook(
+    userIn,
+    {profile: {name: 'Foo Bar'}},
+    userIn
+  );
+
+  test.equal(typeof userOut.createdAt, 'number');
+  test.equal(userOut.profile.name, 'Foo Bar');
+  test.equal(userOut.username, userIn.username);
+
+  // insert the user
+  var uid = Meteor.users.insert(userOut);
+
+  // run the hook again. now the user exists, so it throws an error.
+  test.throws(function () {
+    Meteor.accounts.onCreateUserHook(
+      userIn,
+      {profile: {name: 'Foo Bar'}},
+      userIn
+    );
+  });
+
+  // cleanup
+  Meteor.users.remove(uid);
+
+});
+
+Tinytest.add('accounts - onCreateUserHook email', function (test) {
+  var email1 = Meteor.uuid();
+  var email2 = Meteor.uuid();
+  var email3 = Meteor.uuid();
+  var userIn = {
+    emails: [{address: email1, verified: false},
+             {address: email2, verified: true}]
+  };
+
+  // user does not already exist. return a user object with fields set.
+  var userOut = Meteor.accounts.onCreateUserHook(
+    userIn,
+    {profile: {name: 'Foo Bar'}},
+    userIn
+  );
+
+  test.equal(typeof userOut.createdAt, 'number');
+  test.equal(userOut.profile.name, 'Foo Bar');
+  test.equal(userOut.emails, userIn.emails);
+
+  // insert the user
+  var uid = Meteor.users.insert(userOut);
+
+  // run the hook again with the exact same emails.
+  // run the hook again. now the user exists, so it throws an error.
+  test.throws(function () {
+    Meteor.accounts.onCreateUserHook(
+      userIn,
+      {profile: {name: 'Foo Bar'}},
+      userIn
+    );
+  });
+
+  // now with only one of them.
+  test.throws(function () {
+    Meteor.accounts.onCreateUserHook(
+      {}, {}, {emails: [{address: email1}]}
+    );
+  });
+
+  test.throws(function () {
+    Meteor.accounts.onCreateUserHook(
+      {}, {}, {emails: [{address: email2}]}
+    );
+  });
+
+
+  // a third email works.
+  var user3 = Meteor.accounts.onCreateUserHook(
+      {}, {}, {emails: [{address: email3}]}
+  );
+  test.equal(typeof userOut.createdAt, 'number');
+
+
+  // cleanup
+  Meteor.users.remove(uid);
+
+});