diff --git a/masque/library.py b/masque/library.py
index 9d1f1b7..df637af 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -926,8 +926,13 @@ class ILibrary(ILibraryView, MutableMapping[str, 'Pattern'], metaclass=ABCMeta):
         duplicates = set(self.keys()) & set(other.keys())
 
         if not duplicates:
-            for key in other:
-                self._merge(key, other, key)
+            if mutate_other:
+                temp = other
+            else:
+                temp = Library(copy.deepcopy(dict(other)))
+
+            for key in temp:
+                self._merge(key, temp, key)
             return {}
 
         if mutate_other:
diff --git a/masque/test/test_library.py b/masque/test/test_library.py
index 6ac8536..d035db6 100644
--- a/masque/test/test_library.py
+++ b/masque/test/test_library.py
@@ -221,6 +221,17 @@ def test_library_rename() -> None:
     assert "old" not in lib["parent"].refs
 
 
+def test_library_add_no_duplicates_respects_mutate_other_false() -> None:
+    src_pat = Pattern(ports={"A": Port((0, 0), 0)})
+    lib = Library({"a": Pattern()})
+
+    lib.add({"b": src_pat}, mutate_other=False)
+
+    assert lib["b"] is not src_pat
+    lib["b"].ports["A"].offset[0] = 123
+    assert tuple(src_pat.ports["A"].offset) == (0.0, 0.0)
+
+
 def test_library_subtree() -> None:
     lib = Library()
     lib["a"] = Pattern()