diff --git a/masque/library.py b/masque/library.py
index d7a30af..e98d98d 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -180,6 +180,8 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
 
         if isinstance(tops, str):
             tops = (tops,)
+        tops = set(tops)
+        skip |= tops     # don't re-visit tops
 
         # Get referenced patterns for all tops
         targets = set()