diff --git a/server/internal/infrastructure/marketplace/marketplace.go b/server/internal/infrastructure/marketplace/marketplace.go
index d73bdcdea..06db258f8 100644
--- a/server/internal/infrastructure/marketplace/marketplace.go
+++ b/server/internal/infrastructure/marketplace/marketplace.go
@@ -7,7 +7,6 @@ import (
 	"strings"
 
 	"github.com/reearth/reearth/server/pkg/id"
-	"github.com/reearth/reearth/server/pkg/log"
 	"github.com/reearth/reearth/server/pkg/plugin/pluginpack"
 	"github.com/reearth/reearthx/rerror"
 	"golang.org/x/oauth2/clientcredentials"
@@ -17,21 +16,13 @@ var pluginPackageSizeLimit int64 = 10 * 1024 * 1024 // 10MB
 
 type Marketplace struct {
 	endpoint string
-	client   *http.Client
+	conf     *clientcredentials.Config
 }
 
 func New(endpoint string, conf *clientcredentials.Config) *Marketplace {
-	var client *http.Client
-	if conf != nil {
-		client = conf.Client(context.Background())
-	}
-	if client == nil {
-		client = http.DefaultClient
-	}
-
 	return &Marketplace{
 		endpoint: strings.TrimSuffix(endpoint, "/"),
-		client:   client,
+		conf:     conf,
 	}
 }
 
@@ -103,9 +94,15 @@ type plugin struct {
 */
 
 func (m *Marketplace) downloadPluginPackage(ctx context.Context, url string) (*pluginpack.Package, error) {
-	log.Infof("marketplace: download plugin from \"%s\"", url)
+	var client *http.Client
+	if m.conf != nil {
+		client = m.conf.Client(ctx)
+	}
+	if client == nil {
+		client = http.DefaultClient
+	}
 
-	res, err := m.client.Get(url)
+	res, err := client.Get(url)
 	if err != nil {
 		return nil, rerror.ErrInternalBy(err)
 	}
diff --git a/server/internal/usecase/interactor/plugin_upload.go b/server/internal/usecase/interactor/plugin_upload.go
index 8c496b338..053098dfd 100644
--- a/server/internal/usecase/interactor/plugin_upload.go
+++ b/server/internal/usecase/interactor/plugin_upload.go
@@ -173,7 +173,9 @@ func (i *Plugin) installScenePlugin(ctx context.Context, p *pluginpack.Package,
 		}
 	}
 
-	s.Plugins().Add(scene.NewPlugin(p.Manifest.Plugin.ID(), ppid))
+	if !s.Plugins().Add(scene.NewPlugin(p.Manifest.Plugin.ID(), ppid)) {
+		return interfaces.ErrPluginAlreadyInstalled
+	}
 
 	if pp != nil {
 		if err := i.propertyRepo.Save(ctx, pp); err != nil {
diff --git a/server/internal/usecase/interactor/scene_plugin.go b/server/internal/usecase/interactor/scene_plugin.go
index 7647a6121..093a945da 100644
--- a/server/internal/usecase/interactor/scene_plugin.go
+++ b/server/internal/usecase/interactor/scene_plugin.go
@@ -60,7 +60,9 @@ func (i *Scene) InstallPlugin(ctx context.Context, sid id.SceneID, pid id.Plugin
 		}
 	}
 
-	s.Plugins().Add(scene.NewPlugin(pid, p.IDRef()))
+	if !s.Plugins().Add(scene.NewPlugin(pid, p.IDRef())) {
+		return nil, nil, interfaces.ErrPluginAlreadyInstalled
+	}
 
 	if p != nil {
 		if err := i.propertyRepo.Save(ctx, p); err != nil {
diff --git a/server/pkg/scene/plugins.go b/server/pkg/scene/plugins.go
index 5f664c250..01e1e0228 100644
--- a/server/pkg/scene/plugins.go
+++ b/server/pkg/scene/plugins.go
@@ -67,11 +67,12 @@ func (p *Plugins) HasPluginByName(name string) bool {
 	return false
 }
 
-func (p *Plugins) Add(sp *Plugin) {
+func (p *Plugins) Add(sp *Plugin) bool {
 	if sp == nil || p.HasPluginByName(sp.plugin.Name()) || sp.plugin.Equal(OfficialPluginID) {
-		return
+		return false
 	}
 	p.plugins = append(p.plugins, sp)
+	return true
 }
 
 func (p *Plugins) Remove(pid PluginID) {
diff --git a/server/pkg/scene/plugins_test.go b/server/pkg/scene/plugins_test.go
index a2eb40507..38df11aca 100644
--- a/server/pkg/scene/plugins_test.go
+++ b/server/pkg/scene/plugins_test.go
@@ -317,30 +317,35 @@ func TestPlugins_Add(t *testing.T) {
 		Name         string
 		Input        *Plugin
 		PS, Expected *Plugins
+		Want         bool
 	}{
 		{
 			Name:     "add nil plugin",
 			Input:    nil,
 			PS:       NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
 			Expected: NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
+			Want:     false,
 		},
 		{
 			Name:     "add existing plugin",
 			Input:    NewPlugin(pid, pr),
 			PS:       NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
 			Expected: NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
+			Want:     false,
 		},
 		{
 			Name:     "add official plugin",
 			Input:    NewPlugin(OfficialPluginID, pr),
 			PS:       NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
 			Expected: NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
+			Want:     false,
 		},
 		{
 			Name:     "add new plugin",
 			Input:    NewPlugin(pid, pr),
 			PS:       NewPlugins([]*Plugin{}),
 			Expected: NewPlugins([]*Plugin{NewPlugin(pid, pr)}),
+			Want:     true,
 		},
 	}
 
@@ -348,7 +353,7 @@ func TestPlugins_Add(t *testing.T) {
 		tc := tc
 		t.Run(tc.Name, func(t *testing.T) {
 			t.Parallel()
-			tc.PS.Add(tc.Input)
+			assert.Equal(t, tc.Want, tc.PS.Add(tc.Input))
 			assert.Equal(t, tc.Expected, tc.PS)
 		})
 	}