diff --git a/cmd/gorse-in-one/main.go b/cmd/gorse-in-one/main.go
index f9876bd99..52336bff6 100644
--- a/cmd/gorse-in-one/main.go
+++ b/cmd/gorse-in-one/main.go
@@ -72,7 +72,7 @@ var oneCommand = &cobra.Command{
 			conf.Database.CacheStore = "sqlite://cache.db"
 			conf.Recommend.DataSource.PositiveFeedbackTypes = []string{"star", "like"}
 			conf.Recommend.DataSource.ReadFeedbackTypes = []string{"read"}
-			if err := conf.Validate(true); err != nil {
+			if err := conf.Validate(); err != nil {
 				log.Logger().Fatal("invalid config", zap.Error(err))
 			}
 
diff --git a/config/config.go b/config/config.go
index 9988aad02..d5db32424 100644
--- a/config/config.go
+++ b/config/config.go
@@ -25,6 +25,7 @@ import (
 	"sync"
 	"time"
 
+	mapset "github.com/deckarep/golang-set/v2"
 	"github.com/expr-lang/expr/parser"
 	"github.com/go-playground/locales/en"
 	ut "github.com/go-playground/universal-translator"
@@ -591,7 +592,7 @@ func LoadConfig(path string, oneModel bool) (*Config, error) {
 	}
 
 	// validate config file
-	if err := conf.Validate(oneModel); err != nil {
+	if err := conf.Validate(); err != nil {
 		return nil, errors.Trace(err)
 	}
 
@@ -605,7 +606,25 @@ func LoadConfig(path string, oneModel bool) (*Config, error) {
 	return &conf, nil
 }
 
-func (config *Config) Validate(oneModel bool) error {
+func (config *Config) Validate() error {
+	// Check non-personalized recommenders
+	nonPersonalizedNames := mapset.NewSet[string]()
+	for _, nonPersonalized := range config.Recommend.NonPersonalized {
+		if nonPersonalizedNames.Contains(nonPersonalized.Name) {
+			return errors.Errorf("non-personalized recommender %v is duplicated", nonPersonalized.Name)
+		}
+		nonPersonalizedNames.Add(nonPersonalized.Name)
+	}
+
+	// Check item-to-item recommenders
+	itemToItemNames := mapset.NewSet[string]()
+	for _, itemToItem := range config.Recommend.ItemToItem {
+		if itemToItemNames.Contains(itemToItem.Name) {
+			return errors.Errorf("item-to-item recommender %v is duplicated", itemToItem.Name)
+		}
+		itemToItemNames.Add(itemToItem.Name)
+	}
+
 	validate := validator.New()
 	if err := validate.RegisterValidation("data_store", func(fl validator.FieldLevel) bool {
 		prefixes := []string{
@@ -649,6 +668,10 @@ func (config *Config) Validate(oneModel bool) error {
 		return errors.Trace(err)
 	}
 	if err := validate.RegisterValidation("item_expr", func(fl validator.FieldLevel) bool {
+		if fl.Field().String() == "" {
+			// Empty expression is legal.
+			return true
+		}
 		_, err := parser.Parse(fl.Field().String())
 		return err == nil
 	}); err != nil {
diff --git a/config/config_test.go b/config/config_test.go
index 3d474e642..414646f21 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -26,6 +26,7 @@ import (
 	"github.com/sclevine/yj/convert"
 	"github.com/spf13/viper"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/suite"
 )
 
 func TestUnmarshal(t *testing.T) {
@@ -443,3 +444,40 @@ func TestItemToItemConfig_Hash(t *testing.T) {
 	b = ItemToItemConfig{Column: "b"}
 	assert.NotEqual(t, a.Hash(), b.Hash())
 }
+
+type ValidateTestSuite struct {
+	suite.Suite
+	*Config
+}
+
+func (s *ValidateTestSuite) SetupTest() {
+	s.Config = GetDefaultConfig()
+	s.Database.CacheStore = "redis://localhost:6379/0"
+	s.Database.DataStore = "mysql://gorse:gorse_pass@tcp(localhost:3306)/gorse"
+}
+
+func (s *ValidateTestSuite) TestDuplicateNonPersonalized() {
+	s.Recommend.NonPersonalized = []NonPersonalizedConfig{{
+		Name:  "most_starred_weekly",
+		Score: "count(feedback, .FeedbackType == 'star')",
+	}, {
+		Name:  "most_starred_weekly",
+		Score: "count(feedback, .FeedbackType == 'star')",
+	}}
+	s.Error(s.Validate())
+}
+
+func (s *ValidateTestSuite) TestDuplicateItemToItem() {
+	s.Recommend.ItemToItem = []ItemToItemConfig{{
+		Name: "item_to_item",
+		Type: "users",
+	}, {
+		Name: "item_to_item",
+		Type: "users",
+	}}
+	s.Error(s.Validate())
+}
+
+func TestValidate(t *testing.T) {
+	suite.Run(t, new(ValidateTestSuite))
+}