diff --git a/service.go b/service.go
index e30e0e9..74e06eb 100644
--- a/service.go
+++ b/service.go
@@ -136,12 +136,28 @@ func NewWithLoader(loader Loader, opts ...Option) (*Service, error) {
 			return nil, log.E("NewWithLoader", "load locale: "+lang, err)
 		}
 		lang = normalizeLanguageTag(lang)
-		s.messages[lang] = messages
+		_, seen := s.messages[lang]
+
+		if existing, ok := s.messages[lang]; ok {
+			if existing == nil {
+				existing = make(map[string]Message)
+				s.messages[lang] = existing
+			}
+			maps.Copy(existing, messages)
+		} else {
+			s.messages[lang] = messages
+		}
 		if grammarDataHasContent(grammar) {
-			SetGrammarData(lang, grammar)
+			if seen {
+				MergeGrammarData(lang, grammar)
+			} else {
+				SetGrammarData(lang, grammar)
+			}
 		}
 		tag := language.Make(lang)
-		s.availableLangs = append(s.availableLangs, tag)
+		if !slices.Contains(s.availableLangs, tag) {
+			s.availableLangs = append(s.availableLangs, tag)
+		}
 	}
 
 	if detected := detectLanguage(s.availableLangs); detected != "" {
diff --git a/service_test.go b/service_test.go
index 9b623ad..7220500 100644
--- a/service_test.go
+++ b/service_test.go
@@ -57,6 +57,31 @@ func (underscoreLangLoader) Load(lang string) (map[string]Message, *GrammarData,
 	}, nil, nil
 }
 
+type duplicateLangLoader struct{}
+
+func (duplicateLangLoader) Languages() []string {
+	return []string{"en_US", "en-US"}
+}
+
+func (duplicateLangLoader) Load(lang string) (map[string]Message, *GrammarData, error) {
+	switch lang {
+	case "en_US":
+		return map[string]Message{
+			"first.key": {Text: "first"},
+		}, &GrammarData{
+			Words: map[string]string{"pkg": "package"},
+		}, nil
+	case "en-US":
+		return map[string]Message{
+			"second.key": {Text: "second"},
+		}, &GrammarData{
+			Words: map[string]string{"api": "API"},
+		}, nil
+	default:
+		return map[string]Message{}, nil, nil
+	}
+}
+
 func TestNewService(t *testing.T) {
 	svc, err := New()
 	if err != nil {
@@ -240,6 +265,35 @@ func TestNewWithLoaderNormalisesLanguageTags(t *testing.T) {
 	}
 }
 
+func TestNewWithLoaderMergesCanonicalDuplicateLanguages(t *testing.T) {
+	svc, err := NewWithLoader(duplicateLangLoader{})
+	if err != nil {
+		t.Fatalf("NewWithLoader() failed: %v", err)
+	}
+
+	if err := svc.SetLanguage("en-US"); err != nil {
+		t.Fatalf("SetLanguage(en-US) failed: %v", err)
+	}
+
+	if got, want := svc.T("first.key"), "first"; got != want {
+		t.Fatalf("T(first.key) = %q, want %q", got, want)
+	}
+	if got, want := svc.T("second.key"), "second"; got != want {
+		t.Fatalf("T(second.key) = %q, want %q", got, want)
+	}
+
+	data := GetGrammarData("en-US")
+	if data == nil {
+		t.Fatal("GetGrammarData(en-US) returned nil")
+	}
+	if got, want := data.Words["pkg"], "package"; got != want {
+		t.Fatalf("grammar word pkg = %q, want %q", got, want)
+	}
+	if got, want := data.Words["api"], "API"; got != want {
+		t.Fatalf("grammar word api = %q, want %q", got, want)
+	}
+}
+
 func TestServiceTPluralMessage(t *testing.T) {
 	svc, err := New()
 	if err != nil {