Rely on unique_everseen to avoid unnecessarily polluting the PYTHONPATH with duplicate entries.

setuptools/command/test.py

@@ -15,6 +15,7 @@ from pkg_resources import (resource_listdir, resource_exists, normalize_path,
                            working_set, _namespace_packages, evaluate_marker,
                            add_activation_listener, require, EntryPoint)
 from setuptools import Command
+from .build_py import _unique_everseen
 
 __metaclass__ = type
 
@@ -186,12 +187,11 @@ class test(Command):
         orig_pythonpath = os.environ.get('PYTHONPATH', nothing)
         current_pythonpath = os.environ.get('PYTHONPATH', '')
         try:
-            to_join = []
-            for x in list(paths) + current_pythonpath.split(os.pathsep):
-                if x not in to_join:
-                    to_join.append(x)
-            if to_join:
-                os.environ['PYTHONPATH'] = os.pathsep.join(to_join)
+            prefix = os.pathsep.join(_unique_everseen(paths))
+            to_join = filter(None, [prefix, current_pythonpath])
+            new_path = os.pathsep.join(to_join)
+            if new_path:
+                os.environ['PYTHONPATH'] = new_path
             yield
         finally:
             if orig_pythonpath is nothing: