diff --git a/masque/pattern.py b/masque/pattern.py
index aa585a3..1a54d48 100644
--- a/masque/pattern.py
+++ b/masque/pattern.py
@@ -545,6 +545,8 @@ class Pattern(LockableImpl, AnnotatableImpl, Mirrorable, metaclass=AutoSlots):
     def flatten(self: P) -> P:
         """
         Removes all subpatterns and adds equivalent shapes.
+        Also flattens all subpatterns.
+        Modifies patterns in-place.
 
         Shape/label identifiers are changed to represent their original position in the
          pattern hierarchy:
@@ -555,23 +557,30 @@ class Pattern(LockableImpl, AnnotatableImpl, Mirrorable, metaclass=AutoSlots):
         Returns:
             self
         """
-        subpatterns = copy.deepcopy(self.subpatterns)
-        self.subpatterns = []
+        def flatten_single(pat: P, flattened: Set[P]) -> P:
+            # Update identifiers so each shape has a unique one
+            for ss, shape in enumerate(pat.shapes):
+                shape.identifier = (ss,) + shape.identifier
+            for ll, label in enumerate(pat.labels):
+                label.identifier = (ll,) + label.identifier
 
-        # Update identifiers so each shape has a unique one
-        for ss, shape in enumerate(self.shapes):
-            shape.identifier = (ss,) + shape.identifier
-        for ll, label in enumerate(self.labels):
-            label.identifier = (ll,) + label.identifier
+            for pp, subpat in enumerate(pat.subpatterns):
+                if subpat.pattern is None:
+                    continue
 
-        for pp, subpat in enumerate(subpatterns):
-            if subpat.pattern is None:
-                continue
-            subpat.pattern.flatten()
-            p = subpat.as_pattern()
-            for item in chain(p.shapes, p.labels):
-                item.identifier += (pp,) + item.identifier
-            self.append(p)
+                if subpat.pattern not in flattened:
+                    flatten_single(subpat.pattern, flattened)
+                    flattened.add(subpat.pattern)
+
+                p = subpat.as_pattern()
+                for item in chain(p.shapes, p.labels):
+                    item.identifier = (pp,) + item.identifier
+                pat.append(p)
+
+            pat.subpatterns = []
+            return pat
+
+        flatten_single(self, set())
         return self
 
     def wrap_repeated_shapes(self: P,