diff --git a/src/r_poly.cpp b/src/r_poly.cpp
index 4ea047925e..2e100b3955 100644
--- a/src/r_poly.cpp
+++ b/src/r_poly.cpp
@@ -117,6 +117,8 @@ void PolyRenderer::ClearBuffers()
 	PolyStencilBuffer::Instance()->Clear(RenderTarget->GetWidth(), RenderTarget->GetHeight(), 0);
 	PolySubsectorGBuffer::Instance()->Resize(RenderTarget->GetPitch(), RenderTarget->GetHeight());
 	NextStencilValue = 0;
+	SeenLinePortals.clear();
+	SeenMirrors.clear();
 }
 
 void PolyRenderer::SetSceneViewport()
@@ -170,3 +172,13 @@ void PolyRenderer::SetupPerspectiveMatrix()
 
 	WorldToClip = TriMatrix::perspective(fovy, ratio, 5.0f, 65535.0f) * worldToView;
 }
+
+bool PolyRenderer::InsertSeenLinePortal(FLinePortal *portal)
+{
+	return SeenLinePortals.insert(portal).second;
+}
+
+bool PolyRenderer::InsertSeenMirror(line_t *mirrorLine)
+{
+	return SeenMirrors.insert(mirrorLine).second;
+}
diff --git a/src/r_poly.h b/src/r_poly.h
index bc108a8a13..f34106a4b2 100644
--- a/src/r_poly.h
+++ b/src/r_poly.h
@@ -47,6 +47,9 @@ public:
 	
 	uint32_t GetNextStencilValue() { uint32_t value = NextStencilValue; NextStencilValue += 2; return value; }
 
+	bool InsertSeenLinePortal(FLinePortal *portal);
+	bool InsertSeenMirror(line_t *mirrorLine);
+
 private:
 	void ClearBuffers();
 	void SetSceneViewport();
@@ -57,4 +60,7 @@ private:
 	PolySkyDome Skydome;
 	RenderPolyPlayerSprites PlayerSprites;
 	uint32_t NextStencilValue = 0;
+
+	std::set<FLinePortal *> SeenLinePortals;
+	std::set<line_t *> SeenMirrors;
 };
diff --git a/src/r_poly_wall.cpp b/src/r_poly_wall.cpp
index 384529da08..efc9530138 100644
--- a/src/r_poly_wall.cpp
+++ b/src/r_poly_wall.cpp
@@ -40,24 +40,30 @@ bool RenderPolyWall::RenderLine(const TriMatrix &worldToClip, const Vec4f &clipP
 	PolyDrawLinePortal *polyportal = nullptr;
 	if (line->backsector == nullptr && line->linedef && line->sidedef == line->linedef->sidedef[0] && (line->linedef->special == Line_Mirror && r_drawmirrors))
 	{
-		linePortals.push_back(std::make_unique<PolyDrawLinePortal>(line->linedef));
-		polyportal = linePortals.back().get();
+		if (PolyRenderer::Instance()->InsertSeenMirror(line->linedef))
+		{
+			linePortals.push_back(std::make_unique<PolyDrawLinePortal>(line->linedef));
+			polyportal = linePortals.back().get();
+		}
 	}
 	else if (line->linedef && line->linedef->isVisualPortal())
 	{
 		FLinePortal *portal = line->linedef->getPortal();
-		for (auto &p : linePortals)
+		if (PolyRenderer::Instance()->InsertSeenLinePortal(portal))
 		{
-			if (p->Portal == portal) // To do: what other criterias do we need to check for?
+			for (auto &p : linePortals)
 			{
-				polyportal = p.get();
-				break;
+				if (p->Portal == portal) // To do: what other criterias do we need to check for?
+				{
+					polyportal = p.get();
+					break;
+				}
+			}
+			if (!polyportal)
+			{
+				linePortals.push_back(std::make_unique<PolyDrawLinePortal>(portal));
+				polyportal = linePortals.back().get();
 			}
-		}
-		if (!polyportal)
-		{
-			linePortals.push_back(std::make_unique<PolyDrawLinePortal>(portal));
-			polyportal = linePortals.back().get();
 		}
 	}