diff -r ae76a1046bb9 Lib/site.py
--- a/Lib/site.py	Wed Mar 16 14:30:16 2016 +0100
+++ b/Lib/site.py	Sun Mar 20 12:51:13 2016 +0100
@@ -168,7 +168,7 @@
                     continue
                 line = line.rstrip()
                 dir, dircase = makepath(sitedir, line)
-                if not dircase in known_paths and os.path.exists(dir):
+                if not dircase in known_paths and os.path.exists(dir) and dir not in sys.path:
                     sys.path.append(dir)
                     known_paths.add(dircase)
             except Exception:
@@ -194,7 +194,7 @@
     else:
         reset = 0
     sitedir, sitedircase = makepath(sitedir)
-    if not sitedircase in known_paths:
+    if not sitedircase in known_paths and sitedir not in sys.path:
         sys.path.append(sitedir)        # Add path component
         known_paths.add(sitedircase)
     try: