Fix duplicate references in Scene.mobjects after ReplacementTransform with existing target mobject (#4242)

Co-authored-by: Francisco Manríquez Novoa <49853152+chopan050@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

manim/scene/scene.py

@@ -592,6 +592,19 @@ def replace(self, old_mobject: Mobject, new_mobject: Mobject) -> None:
         def replace_in_list(
             mobj_list: list[Mobject], old_m: Mobject, new_m: Mobject
         ) -> bool:
+            # Avoid duplicate references to the same object in self.mobjects
+            if new_m in mobj_list:
+                if old_m is new_m:
+                    # In this case, one could say that the old Mobject was already found.
+                    # No replacement is needed, since old_m is new_m, so no action is required.
+                    # This might be unexpected, so raise a warning.
+                    logger.warning(
+                        f"Attempted to replace {type(old_m).__name__} "
+                        "with itself in Scene.mobjects."
+                    )
+                    return True
+                mobj_list.remove(new_m)
+
             # We use breadth-first search because some Mobjects get very deep and
             # we expect top-level elements to be the most common targets for replace.
             for i in range(0, len(mobj_list)):

tests/module/animation/test_transform.py

@@ -0,0 +1,27 @@
+from __future__ import annotations
+
+from manim import Circle, ReplacementTransform, Scene, Square, VGroup
+
+
+def test_no_duplicate_references():
+    scene = Scene()
+    c = Circle()
+    sq = Square()
+    scene.add(c, sq)
+
+    scene.play(ReplacementTransform(c, sq))
+    assert len(scene.mobjects) == 1
+    assert scene.mobjects[0] is sq
+
+
+def test_duplicate_references_in_group():
+    scene = Scene()
+    c = Circle()
+    sq = Square()
+    vg = VGroup(c, sq)
+    scene.add(vg)
+
+    scene.play(ReplacementTransform(c, sq))
+    submobs = vg.submobjects
+    assert len(submobs) == 1
+    assert submobs[0] is sq