diff --git a/src/hb-algs.hh b/src/hb-algs.hh
index d63d4f30d..d85a4afe1 100644
--- a/src/hb-algs.hh
+++ b/src/hb-algs.hh
@@ -524,6 +524,18 @@ struct hb_pair_t
   bool operator > (const pair_t& o) const { return first > o.first || (first == o.first && second > o.second); }
   bool operator <= (const pair_t& o) const { return !(*this > o); }
 
+  static int cmp (const void *pa, const void *pb)
+  {
+    pair_t *a = (pair_t *) pa;
+    pair_t *b = (pair_t *) pb;
+
+    if (a->first < b->first) return -1;
+    if (a->first > b->first) return +1;
+    if (a->second < b->second) return -1;
+    if (a->second > b->second) return +1;
+    return 0;
+  }
+
   friend void swap (hb_pair_t& a, hb_pair_t& b)
   {
     hb_swap (a.first, b.first);
diff --git a/src/hb-subset-plan.cc b/src/hb-subset-plan.cc
index 76666ad17..943cf082d 100644
--- a/src/hb-subset-plan.cc
+++ b/src/hb-subset-plan.cc
@@ -541,10 +541,15 @@ _populate_unicodes_to_retain (const hb_set_t *unicodes,
       }
       for (hb_codepoint_t cp : *unicodes)
       {
+	/* Don't double-add entry. */
+	if (plan->codepoint_to_glyph->has (cp))
+	  continue;
+
 	hb_codepoint_t gid = (*unicode_glyphid_map)[cp];
 	plan->codepoint_to_glyph->set (cp, gid);
 	plan->unicode_to_new_gid_list.push (hb_pair (cp, gid));
       }
+      plan->unicode_to_new_gid_list.qsort ();
     }
     else
     {
@@ -571,7 +576,7 @@ _populate_unicodes_to_retain (const hb_set_t *unicodes,
   auto &arr = plan->unicode_to_new_gid_list;
   if (arr.length)
   {
-    plan->unicodes->add_array (&arr.arrayZ->first, arr.length, sizeof (*arr.arrayZ));
+    plan->unicodes->add_sorted_array (&arr.arrayZ->first, arr.length, sizeof (*arr.arrayZ));
     plan->_glyphset_gsub->add_array (&arr.arrayZ->second, arr.length, sizeof (*arr.arrayZ));
   }
 }
diff --git a/src/hb-subset-plan.hh b/src/hb-subset-plan.hh
index 1dea7aad9..0b162ee7b 100644
--- a/src/hb-subset-plan.hh
+++ b/src/hb-subset-plan.hh
@@ -115,7 +115,7 @@ struct hb_subset_plan_t
 
   // For each cp that we'd like to retain maps to the corresponding gid.
   hb_set_t *unicodes;
-  hb_vector_t<hb_pair_t<hb_codepoint_t, hb_codepoint_t>> unicode_to_new_gid_list;
+  hb_sorted_vector_t<hb_pair_t<hb_codepoint_t, hb_codepoint_t>> unicode_to_new_gid_list;
 
   // name_ids we would like to retain
   hb_set_t *name_ids;