diff --git a/src/portal.cpp b/src/portal.cpp
index 0b74359a6..41158e0ee 100644
--- a/src/portal.cpp
+++ b/src/portal.cpp
@@ -1246,6 +1246,7 @@ bool P_CollectConnectedGroups(int startgroup, const DVector3 &position, double u
 		{
 			int othergroup = wsec->GetOppositePortalGroup(sector_t::ceiling);
 			DVector2 pos = Displacements.getOffset(startgroup, othergroup) + position;
+			if (processMask.getBit(othergroup)) break;
 			processMask.setBit(othergroup);
 			out.Add(othergroup | FPortalGroupArray::UPPER);
 			wsec = P_PointInSector(pos);	// get upper sector at the exact spot we want to check and repeat
@@ -1256,6 +1257,7 @@ bool P_CollectConnectedGroups(int startgroup, const DVector3 &position, double u
 		{
 			int othergroup = wsec->GetOppositePortalGroup(sector_t::floor);
 			DVector2 pos = Displacements.getOffset(startgroup, othergroup) + position;
+			if (processMask.getBit(othergroup)) break;
 			processMask.setBit(othergroup);
 			out.Add(othergroup | FPortalGroupArray::LOWER);
 			wsec = P_PointInSector(pos);	// get lower sector at the exact spot we want to check and repeat