diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 4d5f26c..18f1beb 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -5921,7 +5921,17 @@ public sealed class GameWindow : IDisposable
             // clobber entries via Deregister.
             {
                 var setup = _physicsDataCache.GetSetup(entity.SourceGfxObjOrSetupId);
-                if (setup is not null)
+                // ISSUES #83 / Phase A1.6 (2026-05-21): landblock stabs use BSP
+                // collision exclusively (retail CBuildingObj). Skip Setup-derived
+                // CylSphere/Sphere/Radius shadow registrations for stabs to
+                // prevent the same doubled-collision bug A1 fixed for the
+                // mesh-AABB-fallback — but on the Setup path instead. Without
+                // this gate, a stab whose source is 0x02xxxxxx (Setup) with
+                // defined CylSpheres registers BOTH per-part BSP shadows AND a
+                // CylSphere shadow with id = entity.Id, producing an invisible
+                // collision cylinder at the stab origin alongside the correct
+                // BSP walls.
+                if (setup is not null && !_isLandblockStab)
                 {
                     float entScale = entity.Scale > 0f ? entity.Scale : 1f;
                     uint shapeIndex = 0;