Fixes duplicate listing of third party libraries in credits

tools/licenses.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Utility for checking and processing licensing information in third_party
 directories.
 
 Usage: licenses.py <command>
 
@@ skipping 316 matching lines @@
 
 
 def FilterDirsWithFiles(dirs_list, root):
     # If a directory contains no files, assume it's a DEPS directory for a
     # project not used by our current configuration and skip it.
     return [x for x in dirs_list if ContainsFiles(x, root)]
 
 
 def FindThirdPartyDirs(prune_paths, root):
     """Find all third_party directories underneath the source root."""
-    third_party_dirs = []
+    third_party_dirs = set()
     for path, dirs, files in os.walk(root):
         path = path[len(root)+1:]  # Pretty up the path.
 
         if path in prune_paths:
             dirs[:] = []
             continue
 
         # Prune out directories we want to skip.
         # (Note that we loop over PRUNE_DIRS so we're not iterating over a
         # list that we're simultaneously mutating.)
         for skip in PRUNE_DIRS:
             if skip in dirs:
                 dirs.remove(skip)
 
         if os.path.basename(path) == 'third_party':
             # Add all subdirectories that are not marked for skipping.
             for dir in dirs:
                 dirpath = os.path.join(path, dir)
                 if dirpath not in prune_paths:
-                    third_party_dirs.append(dirpath)
+                    third_party_dirs.add(dirpath)
 
             # Don't recurse into any subdirs from here.
             dirs[:] = []
             continue
 
         # Don't recurse into paths in ADDITIONAL_PATHS, like we do with regular
         # third_party/foo paths.
         if path in ADDITIONAL_PATHS:
             dirs[:] = []
 
     for dir in ADDITIONAL_PATHS:
         if dir not in prune_paths:
-            third_party_dirs.append(dir)
+            third_party_dirs.add(dir)
 
     return third_party_dirs
 
 
 def ScanThirdPartyDirs(root=None):
     """Scan a list of directories and report on any problems we find."""
     if root is None:
       root = os.getcwd()
     third_party_dirs = FindThirdPartyDirs(PRUNE_PATHS, root)
     third_party_dirs = FilterDirsWithFiles(third_party_dirs, root)
@@ skipping 80 matching lines @@
     elif command == 'credits':
         if not GenerateCredits():
             return 1
     else:
         print __doc__
         return 1
 
 
 if __name__ == '__main__':
   sys.exit(main())