diff --git a/test_objstorage.py b/test_objstorage.py
index 81c5319..021ce4d 100644
--- a/test_objstorage.py
+++ b/test_objstorage.py
@@ -1,64 +1,85 @@
 # Copyright (C) 2015  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import gzip
 import os
 import shutil
 import tempfile
 import unittest
 
 from io import BytesIO
 from nose.tools import istest
 
-from swh.storage.objstorage import ObjStorage
+from swh.storage.objstorage import ObjStorage, DuplicateObjError
 
 
 class Hashlib(unittest.TestCase):
 
     def setUp(self):
         self.content = b'42\n'
         self.obj_id = 'd81cc0710eb6cf9efd5b920a8453e1e07157b6cd'
         self.obj_steps = ['d8', '1c', 'c0']
         self.obj_relpath = os.path.join(*(self.obj_steps + [self.obj_id]))
         self.tmpdir = tempfile.mkdtemp()
         self.storage = ObjStorage(root=self.tmpdir, depth=3)
 
     def tearDown(self):
         shutil.rmtree(self.tmpdir)
 
     def assertGzipContains(self, gzip_path, content):
         self.assertEqual(gzip.open(gzip_path, 'rb').read(), content)
 
     @istest
     def add_bytes_w_id(self):
         obj_path = os.path.join(self.tmpdir, self.obj_relpath)
         self.storage.add_bytes(self.content, obj_id=self.obj_id)
         self.assertTrue(os.path.isfile(obj_path))
         self.assertEqual(gzip.open(obj_path, 'rb').read(), self.content)
         self.assertGzipContains(obj_path, self.content)
 
     @istest
     def add_bytes_wo_id(self):
         obj_path = os.path.join(self.tmpdir, self.obj_relpath)
         self.storage.add_bytes(self.content)
         self.assertTrue(os.path.isfile(obj_path))
         self.assertGzipContains(obj_path, self.content)
 
     @istest
     def add_file_w_id(self):
         obj_path = os.path.join(self.tmpdir, self.obj_relpath)
         self.storage.add_file(BytesIO(self.content),
                               len(self.content),
                               obj_id=self.obj_id)
         self.assertTrue(os.path.isfile(obj_path))
         self.assertGzipContains(obj_path, self.content)
 
     @istest
     def add_file_wo_id(self):
         obj_path = os.path.join(self.tmpdir, self.obj_relpath)
         self.storage.add_file(BytesIO(self.content),
                               len(self.content))
         self.assertTrue(os.path.isfile(obj_path))
         self.assertGzipContains(obj_path, self.content)
+
+    @istest
+    def add_noclobber(self):
+        self.storage.add_bytes(self.content)
+        with self.assertRaises(DuplicateObjError):
+            self.storage.add_bytes(self.content)
+
+    @istest
+    def add_clobber(self):
+        self.storage.add_bytes(self.content)
+        try:
+            self.storage.add_bytes(self.content, clobber=True)
+        except:
+            self.fail('unexpected exception when clobbering')
+
+    @istest
+    def has(self):
+        self.storage.add_bytes(self.content)
+        self.assertTrue(self.storage.has(self.obj_id))
+        self.assertFalse(self.storage.has(
+            'f1d2d2f924e986ac86fdf7b36c94bcdf32beec15'))