Don't link file again if it's already a correct hard link

Closes gh-6450

IPython/utils/path.py

@@ -554,6 +554,12 b' def link_or_copy(src, dst):'
 
     link_errno = link(src, dst)
     if link_errno == errno.EEXIST:
+        if os.stat(src).st_ino == os.stat(dst).st_ino:
+            # dst is already a hard link to the correct file, so we don't need
+            # to do anything else. If we try to link and rename the file
+            # anyway, we get duplicate files - see http://bugs.python.org/issue21876
+            return
+
         new_dst = dst + "-temp-%04X" %(random.randint(1, 16**4), )
         try:
             link_or_copy(src, new_dst)

IPython/utils/tests/test_path.py

@@ -676,3 +676,12 b' class TestLinkOrCopy(object):'
         dst = self.dst("target")
         path.link_or_copy(self.src, dst)
         self.assert_content_equal(self.src, dst)
+
+    def test_link_twice(self):
+        # Linking the same file twice shouldn't leave duplicates around.
+        # See https://github.com/ipython/ipython/issues/6450
+        dst = self.dst('target')
+        path.link_or_copy(self.src, dst)
+        path.link_or_copy(self.src, dst)
+        self.assert_inode_equal(self.src, dst)
+        nt.assert_equal(sorted(os.listdir(self.tempdir.name)), ['src', 'target'])