diff --git a/src/graph/graph.hh b/src/graph/graph.hh
index b3aef558a..0d6adcb64 100644
--- a/src/graph/graph.hh
+++ b/src/graph/graph.hh
@@ -345,7 +345,9 @@ struct graph_t
     }
   }
 
-  unsigned index_for_offset(unsigned node_idx, const void* offset) const
+  // Finds the object id of the object pointed to by the offset at 'offset'
+  // within object[node_idx].
+  unsigned index_for_offset (unsigned node_idx, const void* offset) const
   {
     const auto& node = object (node_idx);
     if (offset < node.head || offset >= node.tail) return -1;
@@ -360,6 +362,24 @@ struct graph_t
     return -1;
   }
 
+  // Finds the object id of the object pointed to by the offset at 'offset'
+  // within object[node_idx]. Ensures that the returned object is safe to mutate.
+  // That is, if the original child object is shared by parents other than node_idx
+  // it will be duplicated and the duplicate will be returned instead.
+  unsigned mutable_index_for_offset (unsigned node_idx, const void* offset)
+  {
+    unsigned child_idx = index_for_offset (node_idx, offset);
+    auto& child = vertices_[child_idx];
+    for (unsigned p : child.parents)
+    {
+      if (p != node_idx) {
+        return duplicate (node_idx, child_idx);
+      }
+    }
+
+    return child_idx;
+  }
+
 
   /*
    * Assign unique space numbers to each connected subgraph of 24 bit and/or 32 bit offset(s).
diff --git a/src/graph/pairpos-graph.hh b/src/graph/pairpos-graph.hh
index 20e54da44..4cfcd89c5 100644
--- a/src/graph/pairpos-graph.hh
+++ b/src/graph/pairpos-graph.hh
@@ -123,9 +123,10 @@ struct PairPosFormat1 : public OT::Layout::GPOS_impl::PairPosFormat1_3<SmallType
     pairSet.len = count;
     c.graph.vertices_[this_index].obj.tail -= (old_count - count) * SmallTypes::size;
 
-    unsigned coverage_id = c.graph.index_for_offset (this_index, &coverage);
+    unsigned coverage_id = c.graph.mutable_index_for_offset (this_index, &coverage);
     unsigned coverage_size = c.graph.vertices_[coverage_id].table_size ();
     auto& coverage_v = c.graph.vertices_[coverage_id];
+
     Coverage* coverage_table = (Coverage*) coverage_v.obj.head;
     if (!coverage_table->sanitize (coverage_v))
       return false;
@@ -478,9 +479,9 @@ struct PairPosFormat2 : public OT::Layout::GPOS_impl::PairPosFormat2_4<SmallType
         (old_count - count) * split_context.class1_record_size;
 
     unsigned coverage_id =
-        graph.index_for_offset (split_context.this_index, &coverage);
+        graph.mutable_index_for_offset (split_context.this_index, &coverage);
     unsigned class_def_1_id =
-        graph.index_for_offset (split_context.this_index, &classDef1);
+        graph.mutable_index_for_offset (split_context.this_index, &classDef1);
     auto& coverage_v = graph.vertices_[coverage_id];
     auto& class_def_1_v = graph.vertices_[class_def_1_id];
     Coverage* coverage_table = (Coverage*) coverage_v.obj.head;