fix(i18n): make default handlers idempotent

Avoid duplicating the built-in handler chain when WithDefaultHandlers is applied to a service that already has the defaults.

\nCo-Authored-By: Virgil <virgil@lethean.io>

service.go

@@ -5,6 +5,7 @@ import (
 	"io/fs"
 	"maps"
 	"path"
+	"reflect"
 	"slices"
 	"sync"
 	"sync/atomic"
@@ -55,7 +56,14 @@ func WithHandlers(handlers ...KeyHandler) Option {
 
 // WithDefaultHandlers adds the default i18n.* namespace handlers.
 func WithDefaultHandlers() Option {
-	return func(s *Service) { s.handlers = append(s.handlers, DefaultHandlers()...) }
+	return func(s *Service) {
+		for _, handler := range DefaultHandlers() {
+			if hasHandlerType(s.handlers, handler) {
+				continue
+			}
+			s.handlers = append(s.handlers, handler)
+		}
+	}
 }
 
 // WithMode sets the translation mode.
@@ -713,3 +721,13 @@ func (s *Service) autoDetectLanguage() {
 		s.currentLang = detected
 	}
 }
+
+func hasHandlerType(handlers []KeyHandler, candidate KeyHandler) bool {
+	want := reflect.TypeOf(candidate)
+	for _, handler := range handlers {
+		if reflect.TypeOf(handler) == want {
+			return true
+		}
+	}
+	return false
+}

service_test.go

@@ -412,6 +412,17 @@ func TestServiceHandlers(t *testing.T) {
 	}
 }
 
+func TestWithDefaultHandlers_Idempotent(t *testing.T) {
+	svc, err := New(WithDefaultHandlers())
+	if err != nil {
+		t.Fatalf("New() with WithDefaultHandlers() failed: %v", err)
+	}
+
+	if got := len(svc.Handlers()); got != 6 {
+		t.Fatalf("len(Handlers()) = %d, want 6", got)
+	}
+}
+
 func TestServiceWithOptions(t *testing.T) {
 	svc, err := New(
 		WithFallback("en"),