A rapidly growing e-commerce company needs to design a highly scalable and reliable notification system. This system will be responsible for sending various types of notifications (email, push, and SMS) to millions of users globally. The system must handle high throughput, ensure timely delivery, and be resilient to failures.

Design this notification system, addressing the following key components and considerations:

1. API Design: Describe the RESTful API endpoints for external services to trigger notifications. Include:
   • Endpoint paths (e.g., POST /notifications)
   • Request body examples (JSON payload for different notification types, user identifiers, message content, priority, etc.)
   • Response examples (success/failure, notification ID)
   • Error handling strategies (HTTP status codes, error messages).

2. User Preferences Management: Explain how to store and manage user preferences (e.g., opting in/out of specific notification types, frequency caps per user/event, quiet hours) efficiently for millions of users. Discuss database choices, indexing strategies, and caching.

3. Duplicate Notification Prevention: Detail mechanisms to prevent sending duplicate notifications for the same event to the same user across different channels (e.g., avoid sending both an email and an SMS for the same order confirmation unless explicitly allowed) or at different times (e.g., multiple retries resulting in duplicates). Consider idempotency and tracking.

4. Message Queue Implementation: Describe how message queues (e.g., Kafka, RabbitMQ, SQS) would be used to handle high throughput, ensure reliability, implement retry mechanisms, and decouple notification services. Discuss:
   • Choice of message queue and why.
   • Message structure (metadata, payload).
   • Producers (what generates messages).
   • Consumers (how they process messages).
   • Dead-letter queues (purpose and implementation).
   • Guarantees (at-least-once, exactly-once semantics).

5. Overall System Architecture: Provide a high-level architectural diagram (textual description is sufficient; no actual image required) outlining the main components and their interactions. Include:
   • API Gateway
   • Notification Service (main logic, validation)
   • Preference Service
   • Queue(s)
   • Sender Services for Email/Push/SMS
   • Database(s) (for preferences, notification logs, etc.)
   • Caching layers
   • Monitoring and Alerting components.

Throughout your design, discuss trade-offs, scalability, reliability, fault tolerance, and security implications.