diff --git a/setup.py b/setup.py
index c7c5929..d6a5866 100644
--- a/setup.py
+++ b/setup.py
@@ -37,6 +37,16 @@ disabled_module_list = []
 # when running from the build dir, see Modules/getpath.c)
 _BUILDDIR_COOKIE = "pybuilddir.txt"
 
+def append_dir_to_list(dirlist, dir):
+    """Append the directory 'dir' to the list 'dirlist' if:
+
+    1) 'dir' actually exists, and is a directory.
+    2) 'dir' is not already in 'dirlist'
+    """
+    if dir is None or not os.path.isdir(dir) or dir in dirlist:
+        return
+    dirlist.append(dir)
+
 def add_dir_to_list(dirlist, dir):
     """Add the directory 'dir' to the list 'dirlist' (after any relative
     directories) if:
@@ -423,10 +433,10 @@ class PyBuildExt(build_ext):
                             for d in line.strip().split("=")[1].split(":"):
                                 d = os.path.normpath(d)
                                 if '/gcc/' not in d:
-                                    add_dir_to_list(self.compiler.library_dirs,
+                                    append_dir_to_list(self.compiler.library_dirs,
                                                     d)
                         elif is_gcc and in_incdirs and '/gcc/' not in line:
-                            add_dir_to_list(self.compiler.include_dirs,
+                            append_dir_to_list(self.compiler.include_dirs,
                                             line.strip())
         finally:
             os.unlink(tmpfile)
-- 
1.6.4