diff --git a/module/example_test.go b/module/example_test.go
index 43ae5f0..a8246ab 100644
--- a/module/example_test.go
+++ b/module/example_test.go
@@ -7,20 +7,28 @@ import (
 	"github.com/googollee/go-espresso/module"
 )
 
-type DB struct {
+type DB interface {
+	Target() string
+}
+
+type db struct {
 	target string
 }
 
-func NewDB(ctx context.Context) (*DB, error) {
-	return &DB{
+func NewDB(ctx context.Context) (DB, error) {
+	return &db{
 		target: "localhost.db",
 	}, nil
 }
 
+func (db *db) Target() string {
+	return db.target
+}
+
 var ModuleDB = module.New(NewDB)
 
 type Cache struct {
-	fallback *DB
+	fallback DB
 }
 
 func NewCache(ctx context.Context) (*Cache, error) {
@@ -51,8 +59,8 @@ func ExampleModule() {
 	db := ModuleDB.Value(ctx)
 	cache := ModuleCache.Value(ctx)
 
-	fmt.Println("db target:", db.target)
-	fmt.Println("cache fallback target:", cache.fallback.target)
+	fmt.Println("db target:", db.Target())
+	fmt.Println("cache fallback target:", cache.fallback.Target())
 
 	// Output:
 	// db target: localhost.db
@@ -63,9 +71,9 @@ func ExampleModule_withOtherValue() {
 	targetKey := "target"
 	ctx := context.WithValue(context.Background(), targetKey, "target.db")
 
-	newDB := func(ctx context.Context) (*DB, error) {
+	newDB := func(ctx context.Context) (DB, error) {
 		target := ctx.Value(targetKey).(string)
-		return &DB{
+		return &db{
 			target: target,
 		}, nil
 	}
@@ -94,8 +102,8 @@ func ExampleModule_withOtherValue() {
 
 	_ = db
 	_ = cache
-	fmt.Println("db target:", db.target)
-	fmt.Println("cache fallback target:", cache.fallback.target)
+	fmt.Println("db target:", db.Target())
+	fmt.Println("cache fallback target:", cache.fallback.Target())
 
 	// Output:
 	// db target: target.db
@@ -105,8 +113,8 @@ func ExampleModule_withOtherValue() {
 func ExampleModule_createWithError() {
 	ctx := context.Background()
 
-	newDB := func(ctx context.Context) (*DB, error) {
-		return &DB{
+	newDB := func(ctx context.Context) (DB, error) {
+		return &db{
 			target: "localhost.db",
 		}, nil
 	}
@@ -135,8 +143,8 @@ func ExampleModule_createWithError() {
 func ExampleModule_createWithPanic() {
 	ctx := context.Background()
 
-	newDB := func(ctx context.Context) (*DB, error) {
-		return &DB{
+	newDB := func(ctx context.Context) (DB, error) {
+		return &db{
 			target: "localhost.db",
 		}, nil
 	}
@@ -183,3 +191,44 @@ func ExampleModule_notExistingProvider() {
 	// Output:
 	// inject error: module *module_test.Cache creates an instance error: no db as fallback
 }
+
+type FileSystem interface {
+	Read()
+	Write()
+}
+
+type realFileSystem struct{}
+
+func NewRealFileSystem(context.Context) (FileSystem, error) {
+	return &realFileSystem{}, nil
+}
+
+var RealFileSystem = module.New(NewRealFileSystem)
+
+func (f *realFileSystem) Read()  {}
+func (f *realFileSystem) Write() {}
+
+type mockFileSystem struct{}
+
+func NewMockFileSystem(context.Context) (FileSystem, error) {
+	return &mockFileSystem{}, nil
+}
+
+var MockFileSystem = module.New(NewMockFileSystem)
+
+func (f *mockFileSystem) Read()  {}
+func (f *mockFileSystem) Write() {}
+
+func ExampleModule_duplicatingProviders() {
+	defer func() {
+		p := recover()
+		fmt.Println("panic:", p)
+	}()
+
+	repo := module.NewRepo()
+	repo.AddModule(RealFileSystem)
+	repo.AddModule(MockFileSystem)
+
+	// Output:
+	// panic: already exist a provider with type module_test.FileSystem
+}
diff --git a/module/module.go b/module/module.go
index 210b34f..b42e765 100644
--- a/module/module.go
+++ b/module/module.go
@@ -4,7 +4,7 @@ package module
 
 import (
 	"context"
-	"fmt"
+	"reflect"
 )
 
 // BuildFunc is the constructor of an Instance.
@@ -30,7 +30,7 @@ type Module[T any] struct {
 func New[T any](builder BuildFunc[T]) Module[T] {
 	var t T
 	return Module[T]{
-		moduleKey: moduleKey(fmt.Sprintf("%T", t)),
+		moduleKey: moduleKey(reflect.TypeOf(&t).Elem().String()),
 		builder:   builder,
 	}
 }
diff --git a/module/repo.go b/module/repo.go
index 524cf2c..4049d2e 100644
--- a/module/repo.go
+++ b/module/repo.go
@@ -21,6 +21,9 @@ func NewRepo() *Repo {
 // AddModule adds a module to the repo.
 // Module always implements Provider, so a module can be added directly.
 func (r *Repo) AddModule(provider Provider) {
+	if _, ok := r.providers[provider.key()]; ok {
+		panic("already exist a provider with type " + provider.key())
+	}
 	r.providers[provider.key()] = provider
 }