diff --git a/render/render.go b/render/render.go
index 4d426e47c..65e5b6204 100644
--- a/render/render.go
+++ b/render/render.go
@@ -85,7 +85,7 @@ func (m Map) render(rpt report.Report) (RenderableNodes, map[string]string) {
 
 		output[outRenderable.ID] = outRenderable
 		mapped[inRenderable.ID] = outRenderable.ID
-		adjacencies[outRenderable.ID] = adjacencies[outRenderable.ID].Add(inRenderable.Adjacency...)
+		adjacencies[outRenderable.ID] = adjacencies[outRenderable.ID].Merge(inRenderable.Adjacency)
 	}
 
 	// Rewrite Adjacency for new node IDs.
diff --git a/render/renderable_node.go b/render/renderable_node.go
index c5ccad8f4..fec35f9e6 100644
--- a/render/renderable_node.go
+++ b/render/renderable_node.go
@@ -53,8 +53,8 @@ func (rn *RenderableNode) Merge(other RenderableNode) {
 		panic(rn.ID)
 	}
 
-	rn.Adjacency = rn.Adjacency.Add(other.Adjacency...)
-	rn.Origins = rn.Origins.Add(other.Origins...)
+	rn.Adjacency = rn.Adjacency.Merge(other.Adjacency)
+	rn.Origins = rn.Origins.Merge(other.Origins)
 
 	rn.AggregateMetadata.Merge(other.AggregateMetadata)
 	rn.NodeMetadata.Merge(other.NodeMetadata)
diff --git a/report/id_list.go b/report/id_list.go
index 9d89d7d08..6fd9596e1 100644
--- a/report/id_list.go
+++ b/report/id_list.go
@@ -8,6 +8,12 @@ type IDList []string
 // MakeIDList makes a new IDList.
 func MakeIDList(ids ...string) IDList {
 	sort.Strings(ids)
+	for i := 1; i < len(ids); i++ { // shuffle down any duplicates
+		if ids[i-1] == ids[i] {
+			ids = append(ids[:i-1], ids[i:]...)
+			i--
+		}
+	}
 	return IDList(ids)
 }
 
@@ -20,11 +26,41 @@ func (a IDList) Add(ids ...string) IDList {
 			continue
 		}
 		// It a new element, insert it in order.
-		a = append(a[:i], append(IDList{s}, a[i:]...)...)
+		a = append(a, "")
+		copy(a[i+1:], a[i:])
+		a[i] = s
 	}
 	return a
 }
 
+// Merge all elements from a and b into a new list
+func (a IDList) Merge(b IDList) IDList {
+	if len(b) == 0 { // Optimise special case, to avoid allocating
+		return a // (note unit test DeepEquals breaks if we don't do this)
+	}
+	d := make(IDList, len(a)+len(b))
+	for i, j, k := 0, 0, 0; ; k++ {
+		switch {
+		case i >= len(a):
+			copy(d[k:], b[j:])
+			return d[:k+len(b)-j]
+		case j >= len(b):
+			copy(d[k:], a[i:])
+			return d[:k+len(a)-i]
+		case a[i] < b[j]:
+			d[k] = a[i]
+			i++
+		case a[i] > b[j]:
+			d[k] = b[j]
+			j++
+		default: // equal
+			d[k] = a[i]
+			i++
+			j++
+		}
+	}
+}
+
 // Contains returns true if id is in the list.
 func (a IDList) Contains(id string) bool {
 	i := sort.Search(len(a), func(i int) bool { return a[i] >= id })
diff --git a/report/merge.go b/report/merge.go
index 6c1e06d75..c57bec087 100644
--- a/report/merge.go
+++ b/report/merge.go
@@ -24,7 +24,7 @@ func (t *Topology) Merge(other Topology) {
 // Merge merges another Adjacency list into the receiver.
 func (a *Adjacency) Merge(other Adjacency) {
 	for addr, adj := range other {
-		(*a)[addr] = (*a)[addr].Add(adj...)
+		(*a)[addr] = (*a)[addr].Merge(adj)
 	}
 }