diff --git a/kad/trie/trie.go b/kad/trie/trie.go
index 588d235..b76cca3 100644
--- a/kad/trie/trie.go
+++ b/kad/trie/trie.go
@@ -129,7 +129,7 @@ func (tr *Trie[K, D]) addAtDepth(depth int, kk K, data D) bool {
 	}
 }
 
-// Add adds the key to trie, returning a new trie.
+// Add adds the key to trie, returning a new trie if the key was not already in the trie.
 // Add is immutable/non-destructive: the original trie remains unchanged.
 func Add[K kad.Key[K], D any](tr *Trie[K, D], kk K, data D) (*Trie[K, D], error) {
 	return addAtDepth(0, tr, kk, data), nil
@@ -148,8 +148,12 @@ func addAtDepth[K kad.Key[K], D any](depth int, tr *Trie[K, D], kk K, data D) *T
 
 	default:
 		dir := kk.Bit(depth)
+		b := addAtDepth(depth+1, tr.branch[dir], kk, data)
+		if b == tr.branch[dir] {
+			return tr
+		}
 		s := &Trie[K, D]{}
-		s.branch[dir] = addAtDepth(depth+1, tr.branch[dir], kk, data)
+		s.branch[dir] = b
 		s.branch[1-dir] = tr.branch[1-dir]
 		return s
 	}
diff --git a/kad/trie/trie_test.go b/kad/trie/trie_test.go
index d7acd65..541c37c 100644
--- a/kad/trie/trie_test.go
+++ b/kad/trie/trie_test.go
@@ -167,6 +167,22 @@ func TestSize(t *testing.T) {
 	require.Equal(t, len(sampleKeySet.Keys), tr.Size())
 }
 
+func TestImmutableAddReturnsNewTrie(t *testing.T) {
+	tr := New[kadtest.Key32, any]()
+	for _, kk := range sampleKeySet.Keys {
+		var err error
+		trBefore := *tr // take a copy of tr before Add is called
+		trNext, err := Add(tr, kk, nil)
+		require.NoError(t, err)
+		// a new trie must be returned
+		require.NotSame(t, tr, trNext)
+		// old trie must be not be modified
+		require.EqualValues(t, trBefore, *tr)
+		tr = trNext
+	}
+	require.Equal(t, len(sampleKeySet.Keys), tr.Size())
+}
+
 func TestAddIgnoresDuplicates(t *testing.T) {
 	tr := New[kadtest.Key32, any]()
 	for _, kk := range sampleKeySet.Keys {
@@ -206,6 +222,28 @@ func TestImmutableAddIgnoresDuplicates(t *testing.T) {
 	}
 }
 
+func TestImmutableAddReturnsOriginalTrieForDuplicates(t *testing.T) {
+	tr := New[kadtest.Key32, any]()
+	var err error
+	for _, kk := range sampleKeySet.Keys {
+		tr, err = Add(tr, kk, nil)
+		require.NoError(t, err)
+	}
+	require.Equal(t, len(sampleKeySet.Keys), tr.Size())
+
+	for _, kk := range sampleKeySet.Keys {
+		next, err := Add(tr, kk, nil)
+		require.NoError(t, err)
+		// trie has not been changed
+		require.Same(t, tr, next)
+	}
+	require.Equal(t, len(sampleKeySet.Keys), tr.Size())
+
+	if d := CheckInvariant(tr); d != nil {
+		t.Fatalf("reordered trie invariant discrepancy: %v", d)
+	}
+}
+
 func TestAddWithData(t *testing.T) {
 	tr := New[kadtest.Key32, int]()
 	for i, kk := range sampleKeySet.Keys {
@@ -223,10 +261,13 @@ func TestAddWithData(t *testing.T) {
 
 func TestImmutableAddWithData(t *testing.T) {
 	tr := New[kadtest.Key32, int]()
-	var err error
 	for i, kk := range sampleKeySet.Keys {
-		tr, err = Add(tr, kk, i)
+		var err error
+		trNext, err := Add(tr, kk, i)
 		require.NoError(t, err)
+		// a new trie must be returned
+		require.NotSame(t, tr, trNext)
+		tr = trNext
 	}
 	require.Equal(t, len(sampleKeySet.Keys), tr.Size())
 
@@ -264,6 +305,9 @@ func TestImmutableRemove(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, len(sampleKeySet.Keys)-1, trNext.Size())
 
+	// a new trie must be returned
+	require.NotSame(t, tr, trNext)
+
 	if d := CheckInvariant(tr); d != nil {
 		t.Fatalf("reordered trie invariant discrepancy: %v", d)
 	}
diff --git a/kad/triert/table_test.go b/kad/triert/table_test.go
index caf1f80..a2268a0 100644
--- a/kad/triert/table_test.go
+++ b/kad/triert/table_test.go
@@ -321,7 +321,8 @@ type nodeFilter struct {
 }
 
 func (f *nodeFilter) TryAdd(rt *TrieRT[kadtest.Key32, node[kadtest.Key32]],
-	n node[kadtest.Key32]) bool {
+	n node[kadtest.Key32],
+) bool {
 	if n == node2 {
 		return false
 	}