ifcgeom: keep one copy of repeated faces in faceset helper duplicate removal

When an IfcConnectedFaceSet lists the same IfcFace instance multiple
times, the faceset helper registered the loop identity in duplicates_
and wires() then skipped every occurrence of that identity, so the face
vanished from the shell entirely instead of being deduplicated to a
single copy. The resulting shell had holes, sewing produced a
non-manifold first/second boolean operand, and opening subtraction was
discarded (Boolean operation yields non-manifold result), leaving the
element with no geometry.

Count how many occurrences of a loop identity are redundant and skip
only that many in wires(), so exactly one copy of each repeated face is
still built.

In 418--walls--segfault.ifc the opening of wall #2543 repeats 84 of the
350 unique faces of its closed shell (once to three times over the 518
entries), which previously dropped those 84 faces and yielded a 0-vertex
wall. With this change the opening operand is manifold and the wall gets
a closed, dimensionally correct body. No output changes on the other 257
files in test/input.

Generated with the assistance of an AI coding tool.

src/ifcgeom/kernels/opencascade/OpenCascadeKernel.h

@@ -86,6 +86,7 @@ private:
 	private:
 		OpenCascadeKernel* kernel_;
 		std::set<int> duplicates_;
+		std::map<int, int> duplicate_skips_remaining_;
 		std::map<int, int> vertex_mapping_;
 		std::map<std::pair<int, int>, TopoDS_Edge> edges_;
 		double eps_;

src/ifcgeom/kernels/opencascade/faceset_helper.cpp

@@ -108,6 +108,7 @@ IfcGeom::OpenCascadeKernel::faceset_helper::faceset_helper(
 
 		vertex_mapping_.clear();
 		duplicates_.clear();
+		duplicate_skips_remaining_.clear();
 
 		edge_use.clear();
 
@@ -174,6 +175,7 @@ IfcGeom::OpenCascadeKernel::faceset_helper::faceset_helper(
 			if (edge_sets.find({loop->external.get_value_or(false), segment_set}) != edge_sets.end()) {
 				duplicate_faces++;
 				duplicates_.insert(loop->identity());
+				duplicate_skips_remaining_[loop->identity()]++;
 				continue;
 			}
             edge_sets.insert({loop->external.get_value_or(false), segment_set});
@@ -250,7 +252,10 @@ bool IfcGeom::OpenCascadeKernel::faceset_helper::wire(const ifcopenshell::geomet
 }
 
 bool IfcGeom::OpenCascadeKernel::faceset_helper::wires(const ifcopenshell::geometry::taxonomy::loop::ptr loop, NCollection_List<TopoDS_Shape>& wires) {
-	if (duplicates_.find(loop->identity()) != duplicates_.end()) {
+	// Skip only the redundant occurrences, keep one copy of the face.
+	auto it = duplicate_skips_remaining_.find(loop->identity());
+	if (it != duplicate_skips_remaining_.end() && it->second > 0) {
+		--it->second;
 		return false;
 	}
 	TopoDS_Wire wire;