[heap] Only insert new slot set entries.

src/heap/slot-set.h

Index: src/heap/slot-set.h
diff --git a/src/heap/slot-set.h b/src/heap/slot-set.h
index 6ba3174e3c42f71198bb851faa2de13cf1bd65ee..de732db454887a92817d01063a9978f9c18041ba 100644
--- a/src/heap/slot-set.h
+++ b/src/heap/slot-set.h
@@ -48,7 +48,9 @@ class SlotSet : public Malloced {
       current_bucket = AllocateBucket();
       bucket[bucket_index].SetValue(current_bucket);
     }
-    current_bucket[cell_index].SetBit(bit_index);
+    if (!(current_bucket[cell_index].Value() & (1u << bit_index))) {
+      current_bucket[cell_index].SetBit(bit_index);
+    }
   }
 
   // The slot offset specifies a slot at address page_start_ + slot_offset.