diff --git a/src/heap_iter.rs b/src/heap_iter.rs
index 032ce6bd..b8fd9dae 100644
--- a/src/heap_iter.rs
+++ b/src/heap_iter.rs
@@ -86,16 +86,22 @@ impl<'a> EagerStackfulPreOrderHeapIter<'a> {
                     let arity = cell_as_atom_cell!(self.heap[s]).get_arity();
 
                     for idx in (s + 1 .. s + arity + 1).rev() {
-                        self.iter_stack.push(self.heap[idx]);
-                        self.heap[idx].set_mark_bit(self.mark_phase);
+			if self.heap[idx].get_mark_bit() != self.mark_phase {
+                            self.iter_stack.push(self.heap[idx]);
+                            self.heap[idx].set_mark_bit(self.mark_phase);
+			}
                     }
                 }
                 (HeapCellValueTag::Lis, l) => {
-                    self.iter_stack.push(self.heap[l+1]);
-                    self.iter_stack.push(self.heap[l]);
+		    if self.heap[l+1].get_mark_bit() != self.mark_phase {
+			self.iter_stack.push(self.heap[l+1]);
+			self.heap[l+1].set_mark_bit(self.mark_phase);
+		    }
 
-                    self.heap[l].set_mark_bit(self.mark_phase);
-                    self.heap[l+1].set_mark_bit(self.mark_phase);
+		    if self.heap[l].get_mark_bit() != self.mark_phase {
+			self.iter_stack.push(self.heap[l]);
+			self.heap[l].set_mark_bit(self.mark_phase);
+		    }
                 }
                 (HeapCellValueTag::AttrVar | HeapCellValueTag::Var, h) => {
                     let var_value = self.heap[h];
diff --git a/src/tests/term_variables.pl b/src/tests/term_variables.pl
index dce49c83..f3ef5b32 100644
--- a/src/tests/term_variables.pl
+++ b/src/tests/term_variables.pl
@@ -45,7 +45,7 @@ test("term_variables#2097", (
 
 test("term_variables#2100", (
     termt2(T), term_variables(T,Vs),
-    T = [[T|_B]|_A], Vs == [_A,_B]
+    T = [[T|A]|B], Vs == [A,B]
 )).
 
 test("term_variables#2101", (