diff --git a/listener/src/services/subscription-service.test.ts b/listener/src/services/subscription-service.test.ts
new file mode 100644
index 0000000..5fa352c
--- /dev/null
+++ b/listener/src/services/subscription-service.test.ts
@@ -0,0 +1,224 @@
+import { SubscriptionService } from './subscription-service';
+import { SubscribeInput } from '../types/subscription';
+
+describe('SubscriptionService', () => {
+  let service: SubscriptionService;
+
+  beforeEach(() => {
+    service = new SubscriptionService();
+  });
+
+  describe('subscribe', () => {
+    it('creates a new subscription successfully', () => {
+      const input: SubscribeInput = {
+        userId: 'user-1',
+        channel: 'discord',
+      };
+
+      const result = service.subscribe(input);
+
+      expect(result.success).toBe(true);
+      expect(result.subscription).toBeDefined();
+      expect(result.subscription?.userId).toBe('user-1');
+      expect(result.subscription?.channel).toBe('discord');
+      expect(result.subscription?.active).toBe(true);
+      expect(result.error).toBeUndefined();
+    });
+
+    it('rejects duplicate subscriptions to the same channel', () => {
+      const input: SubscribeInput = {
+        userId: 'user-1',
+        channel: 'discord',
+      };
+
+      // First subscription succeeds
+      const firstResult = service.subscribe(input);
+      expect(firstResult.success).toBe(true);
+      expect(firstResult.subscription).toBeDefined();
+
+      const originalSubscription = firstResult.subscription;
+
+      // Second subscription to same channel fails
+      const duplicateResult = service.subscribe(input);
+      expect(duplicateResult.success).toBe(false);
+      expect(duplicateResult.error).toBe('DUPLICATE_SUBSCRIPTION');
+      expect(duplicateResult.message).toContain('already subscribed');
+      expect(duplicateResult.subscription).toEqual(originalSubscription);
+    });
+
+    it('allows the same user to subscribe to different channels', () => {
+      const discordResult = service.subscribe({
+        userId: 'user-1',
+        channel: 'discord',
+      });
+
+      const emailResult = service.subscribe({
+        userId: 'user-1',
+        channel: 'email',
+      });
+
+      expect(discordResult.success).toBe(true);
+      expect(emailResult.success).toBe(true);
+      expect(service.getUserSubscriptions('user-1')).toHaveLength(2);
+    });
+
+    it('allows different users to subscribe to the same channel', () => {
+      const user1Result = service.subscribe({
+        userId: 'user-1',
+        channel: 'discord',
+      });
+
+      const user2Result = service.subscribe({
+        userId: 'user-2',
+        channel: 'discord',
+      });
+
+      expect(user1Result.success).toBe(true);
+      expect(user2Result.success).toBe(true);
+      expect(service.count()).toBe(2);
+    });
+
+    it('validates required input fields', () => {
+      const invalidInputs = [
+        { userId: '', channel: 'discord' },
+        { userId: 'user-1', channel: '' },
+        { userId: '', channel: '' },
+      ];
+
+      for (const input of invalidInputs) {
+        const result = service.subscribe(input);
+        expect(result.success).toBe(false);
+        expect(result.error).toBe('INVALID_INPUT');
+        expect(result.subscription).toBeNull();
+      }
+    });
+
+    it('preserves the original subscription when rejecting duplicates', () => {
+      const input: SubscribeInput = {
+        userId: 'user-1',
+        channel: 'discord',
+      };
+
+      // Create initial subscription
+      const firstResult = service.subscribe(input);
+      const originalId = firstResult.subscription!.id;
+      const originalCreatedAt = firstResult.subscription!.createdAt;
+
+      // Wait a bit to ensure timestamps would differ
+      const later = Date.now() + 100;
+      jest.spyOn(Date, 'now').mockReturnValue(later);
+
+      // Attempt duplicate subscription
+      const duplicateResult = service.subscribe(input);
+
+      // Verify the returned subscription is the original, unchanged
+      expect(duplicateResult.subscription?.id).toBe(originalId);
+      expect(duplicateResult.subscription?.createdAt).toBe(originalCreatedAt);
+      expect(duplicateResult.subscription?.createdAt).not.toBe(later);
+
+      jest.restoreAllMocks();
+    });
+  });
+
+  describe('isSubscribed', () => {
+    it('returns true for existing active subscription', () => {
+      service.subscribe({ userId: 'user-1', channel: 'discord' });
+      expect(service.isSubscribed('user-1', 'discord')).toBe(true);
+    });
+
+    it('returns false for non-existent subscription', () => {
+      expect(service.isSubscribed('user-1', 'email')).toBe(false);
+    });
+  });
+
+  describe('getUserSubscriptions', () => {
+    it('returns all subscriptions for a user', () => {
+      service.subscribe({ userId: 'user-1', channel: 'discord' });
+      service.subscribe({ userId: 'user-1', channel: 'email' });
+      service.subscribe({ userId: 'user-2', channel: 'discord' });
+
+      const subscriptions = service.getUserSubscriptions('user-1');
+      expect(subscriptions).toHaveLength(2);
+      expect(subscriptions.map((s) => s.channel)).toContain('discord');
+      expect(subscriptions.map((s) => s.channel)).toContain('email');
+    });
+
+    it('returns empty array for user with no subscriptions', () => {
+      const subscriptions = service.getUserSubscriptions('user-999');
+      expect(subscriptions).toEqual([]);
+    });
+  });
+
+  describe('getSubscription', () => {
+    it('retrieves specific subscription by user and channel', () => {
+      service.subscribe({ userId: 'user-1', channel: 'discord' });
+
+      const subscription = service.getSubscription('user-1', 'discord');
+      expect(subscription).toBeDefined();
+      expect(subscription?.userId).toBe('user-1');
+      expect(subscription?.channel).toBe('discord');
+    });
+
+    it('returns null for non-existent subscription', () => {
+      const subscription = service.getSubscription('user-1', 'email');
+      expect(subscription).toBeNull();
+    });
+  });
+
+  describe('unsubscribe', () => {
+    it('removes an existing subscription', () => {
+      service.subscribe({ userId: 'user-1', channel: 'discord' });
+      expect(service.isSubscribed('user-1', 'discord')).toBe(true);
+
+      const removed = service.unsubscribe('user-1', 'discord');
+      expect(removed).toBe(true);
+      expect(service.isSubscribed('user-1', 'discord')).toBe(false);
+    });
+
+    it('allows re-subscription after unsubscribing', () => {
+      // Initial subscription
+      const first = service.subscribe({ userId: 'user-1', channel: 'discord' });
+      expect(first.success).toBe(true);
+
+      // Unsubscribe
+      service.unsubscribe('user-1', 'discord');
+
+      // Re-subscribe should succeed (not a duplicate since it was removed)
+      const second = service.subscribe({ userId: 'user-1', channel: 'discord' });
+      expect(second.success).toBe(true);
+      expect(second.subscription?.id).not.toBe(first.subscription?.id);
+    });
+
+    it('returns false when unsubscribing non-existent subscription', () => {
+      const removed = service.unsubscribe('user-1', 'email');
+      expect(removed).toBe(false);
+    });
+  });
+
+  describe('count', () => {
+    it('returns correct subscription count', () => {
+      expect(service.count()).toBe(0);
+
+      service.subscribe({ userId: 'user-1', channel: 'discord' });
+      expect(service.count()).toBe(1);
+
+      service.subscribe({ userId: 'user-1', channel: 'email' });
+      expect(service.count()).toBe(2);
+
+      service.unsubscribe('user-1', 'discord');
+      expect(service.count()).toBe(1);
+    });
+  });
+
+  describe('clear', () => {
+    it('removes all subscriptions', () => {
+      service.subscribe({ userId: 'user-1', channel: 'discord' });
+      service.subscribe({ userId: 'user-2', channel: 'email' });
+      expect(service.count()).toBe(2);
+
+      service.clear();
+      expect(service.count()).toBe(0);
+      expect(service.getUserSubscriptions('user-1')).toEqual([]);
+    });
+  });
+});
diff --git a/listener/src/services/subscription-service.ts b/listener/src/services/subscription-service.ts
new file mode 100644
index 0000000..b87f694
--- /dev/null
+++ b/listener/src/services/subscription-service.ts
@@ -0,0 +1,129 @@
+import { Subscription, SubscribeInput, SubscribeResult } from '../types/subscription';
+
+/**
+ * SubscriptionService manages user subscriptions to notification channels.
+ * 
+ * Core responsibilities:
+ * - Prevent duplicate subscriptions (same user + channel)
+ * - Track active subscriptions
+ * - Provide query interface for subscription state
+ * 
+ * This service implements duplicate detection at the application layer,
+ * ensuring users cannot subscribe twice to the same channel.
+ */
+export class SubscriptionService {
+  private subscriptions = new Map<string, Subscription>();
+
+  /**
+   * Generate a unique fingerprint for a subscription.
+   * Format: "userId:channel" (e.g., "user-123:discord")
+   */
+  private getFingerprint(userId: string, channel: string): string {
+    return `${userId}:${channel}`;
+  }
+
+  /**
+   * Generate a unique subscription ID.
+   */
+  private generateId(): string {
+    return `sub_${Date.now()}_${Math.random().toString(36).substring(2, 9)}`;
+  }
+
+  /**
+   * Subscribe a user to a notification channel.
+   * 
+   * Returns existing subscription if already subscribed (idempotent duplicate rejection).
+   * 
+   * @param input - User ID and channel to subscribe to
+   * @returns SubscribeResult with success flag and subscription or error
+   */
+  subscribe(input: SubscribeInput): SubscribeResult {
+    const { userId, channel } = input;
+
+    // Validate input
+    if (!userId || !channel) {
+      return {
+        success: false,
+        subscription: null,
+        error: 'INVALID_INPUT',
+        message: 'userId and channel are required',
+      };
+    }
+
+    const fingerprint = this.getFingerprint(userId, channel);
+
+    // Check for duplicate subscription
+    if (this.subscriptions.has(fingerprint)) {
+      const existing = this.subscriptions.get(fingerprint)!;
+      return {
+        success: false,
+        subscription: existing,
+        error: 'DUPLICATE_SUBSCRIPTION',
+        message: `User ${userId} is already subscribed to channel ${channel}`,
+      };
+    }
+
+    // Create new subscription
+    const subscription: Subscription = {
+      id: this.generateId(),
+      userId,
+      channel,
+      createdAt: Date.now(),
+      active: true,
+    };
+
+    this.subscriptions.set(fingerprint, subscription);
+
+    return {
+      success: true,
+      subscription,
+    };
+  }
+
+  /**
+   * Check if a user is subscribed to a specific channel.
+   */
+  isSubscribed(userId: string, channel: string): boolean {
+    const fingerprint = this.getFingerprint(userId, channel);
+    const subscription = this.subscriptions.get(fingerprint);
+    return !!subscription && subscription.active;
+  }
+
+  /**
+   * Get all active subscriptions for a user.
+   */
+  getUserSubscriptions(userId: string): Subscription[] {
+    return Array.from(this.subscriptions.values())
+      .filter((sub) => sub.userId === userId && sub.active);
+  }
+
+  /**
+   * Get a specific subscription by user and channel.
+   */
+  getSubscription(userId: string, channel: string): Subscription | null {
+    const fingerprint = this.getFingerprint(userId, channel);
+    return this.subscriptions.get(fingerprint) || null;
+  }
+
+  /**
+   * Unsubscribe a user from a channel.
+   */
+  unsubscribe(userId: string, channel: string): boolean {
+    const fingerprint = this.getFingerprint(userId, channel);
+    return this.subscriptions.delete(fingerprint);
+  }
+
+  /**
+   * Get total subscription count (for testing/debugging).
+   */
+  count(): number {
+    return this.subscriptions.size;
+  }
+
+  /**
+   * Clear all subscriptions (for testing).
+   */
+  clear(): void {
+    this.subscriptions.clear();
+  }
+}
diff --git a/listener/src/types/subscription.ts b/listener/src/types/subscription.ts
new file mode 100644
index 0000000..ed5b7a6
--- /dev/null
+++ b/listener/src/types/subscription.ts
@@ -0,0 +1,27 @@
+/**
+ * Subscription types for notification channel management.
+ * 
+ * Subscriptions allow users to explicitly opt-in to specific notification channels.
+ * Unlike preferences (which toggle existing subscriptions), subscriptions represent
+ * active registrations that can be created, queried, and removed.
+ */
+
+export interface Subscription {
+  id: string;
+  userId: string;
+  channel: string; // 'discord', 'email', 'telegram', etc.
+  createdAt: number;
+  active: boolean;
+}
+
+export interface SubscribeInput {
+  userId: string;
+  channel: string;
+}
+
+export interface SubscribeResult {
+  success: boolean;
+  subscription: Subscription | null;
+  error?: 'DUPLICATE_SUBSCRIPTION' | 'INVALID_INPUT';
+  message?: string;
+}