Fixes duplicate listing of third party libraries in credits

tools/licenses.py

Index: tools/licenses.py
diff --git a/tools/licenses.py b/tools/licenses.py
index e87a54d1682565088f01b458b7d5f7d18eda111c..420ece43758ca54c166e87fe223d972583d242fe 100755
--- a/tools/licenses.py
+++ b/tools/licenses.py
@@ -334,7 +334,7 @@ def FilterDirsWithFiles(dirs_list, 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.
 
@@ -354,7 +354,7 @@ def FindThirdPartyDirs(prune_paths, root):
             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[:] = []
@@ -367,7 +367,7 @@ def FindThirdPartyDirs(prune_paths, root):
 
     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