fix tests for changes

src/zc/buildout/easy_install.txt

@@ -1291,6 +1291,7 @@ this is very straightforward.
             ...
             ]
         for path in original_paths:
+            if path not in known_paths:
                 addsitedir(path, known_paths)
         return known_paths
     <BLANKLINE>
@@ -1368,6 +1369,7 @@ call to another text fixture to create.
             ...
             ]
         for path in original_paths:
+            if path not in known_paths:
                 addsitedir(path, known_paths)
         return known_paths
     <BLANKLINE>
@@ -1432,6 +1434,7 @@ at that result.
             ...
             ]
         for path in original_paths:
+            if path not in known_paths:
                 addsitedir(path, known_paths)
         return known_paths
     <BLANKLINE>