diff --git a/internal/session/memory_store.go b/internal/session/memory_store.go
index 7fba7ad..293e228 100644
--- a/internal/session/memory_store.go
+++ b/internal/session/memory_store.go
@@ -50,13 +50,18 @@ func (ms *MemoryStore) Set(s *Session) error {
 	ms.mu.Lock()
 	defer ms.mu.Unlock()
 
-	// 如果 sid 已存在且 uid 不同，先从旧 uid 索引移除
-	if old, ok := ms.sessions[s.ID]; ok && old.UserID != s.UserID {
-		ms.removeFromUIDIndex(old.UserID, s.ID)
+	if old, ok := ms.sessions[s.ID]; ok {
+		// sid 已存在：仅当 uid 变更时更新索引
+		if old.UserID != s.UserID {
+			ms.removeFromUIDIndex(old.UserID, s.ID)
+			ms.byUID[s.UserID] = append(ms.byUID[s.UserID], s.ID)
+		}
+	} else {
+		// 新 sid：追加到索引
+		ms.byUID[s.UserID] = append(ms.byUID[s.UserID], s.ID)
 	}
 
 	ms.sessions[s.ID] = s
-	ms.byUID[s.UserID] = append(ms.byUID[s.UserID], s.ID)
 	return nil
 }