feat: implement Story 4.3 - Returning Participant Detection

Prevent duplicate registrations for the same exchange:

- Check for existing participant with same email (case-insensitive)
- Show friendly message if already registered
- Offer link to request new magic link for access
- Allow same email to register for different exchanges
- Comprehensive test suite with 4 tests

All tests passing (90 total), 91% coverage maintained.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/routes/participant.py

@@ -62,6 +62,26 @@ def register(slug: str):
         # If explicitly unchecked, it will be False
         reminder_enabled = form.reminder_enabled.data
 
+        # Check if participant with this email already exists for this exchange
+        existing_participant = (
+            db.session.query(Participant)
+            .filter_by(exchange_id=exchange.id, email=email)
+            .first()
+        )
+
+        if existing_participant:
+            # Participant already registered
+            flash(
+                "You're already registered for this exchange. "
+                "If you need access, you can request a new magic link below.",
+                "info",
+            )
+            return render_template(
+                "participant/register.html",
+                exchange=exchange,
+                form=form,
+            )
+
         # Create participant record
         participant = Participant(
             exchange_id=exchange.id,

tests/integration/test_story_4_3_returning_participant_detection.py

@@ -0,0 +1,214 @@
+"""Tests for Story 4.3: Returning Participant Detection.
+
+As a potential participant who has already registered, I want to be notified
+if I try to register again so that I don't create duplicate entries.
+"""
+
+from datetime import datetime, timedelta
+
+from src.models.exchange import Exchange
+from src.models.participant import Participant
+
+
+class TestStory43ReturningParticipantDetection:
+    """Test suite for Story 4.3: Returning Participant Detection."""
+
+    def test_duplicate_email_shows_message(self, client, db):
+        """Test that registering with existing email shows message."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="winter2025",
+            name="Office Secret Santa 2025",
+            budget="$50",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.flush()
+
+        # Create existing participant
+        existing_participant = Participant(
+            exchange_id=exchange.id,
+            name="John Doe",
+            email="john.doe@example.com",
+            gift_ideas="Books",
+            reminder_enabled=True,
+        )
+        db.session.add(existing_participant)
+        db.session.commit()
+
+        # Try to register again with same email (case insensitive)
+        response = client.post(
+            "/exchange/winter2025/register",
+            data={
+                "name": "John Doe",
+                "email": "John.Doe@Example.COM",  # Different case
+                "gift_ideas": "Gadgets",
+            },
+            follow_redirects=True,
+        )
+
+        # Should show message that user is already registered
+        assert response.status_code == 200
+        assert b"already registered" in response.data
+
+        # Should not create duplicate participant
+        participants = (
+            db.session.query(Participant).filter_by(email="john.doe@example.com").all()
+        )
+        assert len(participants) == 1
+
+    def test_duplicate_detection_offers_magic_link_request(self, client, db):
+        """Test that duplicate detection page offers link to request magic link."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="test123",
+            name="Test Exchange",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.flush()
+
+        # Create existing participant
+        participant = Participant(
+            exchange_id=exchange.id,
+            name="Jane Smith",
+            email="jane@example.com",
+            reminder_enabled=True,
+        )
+        db.session.add(participant)
+        db.session.commit()
+
+        # Try to register again
+        response = client.post(
+            "/exchange/test123/register",
+            data={
+                "name": "Jane Smith",
+                "email": "jane@example.com",
+            },
+        )
+
+        # Should show link to request access
+        assert (
+            b"request a new magic link" in response.data
+            or b"request access" in response.data
+        )
+
+    def test_different_emails_can_register(self, client, db):
+        """Test that different emails can register successfully."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        exchange = Exchange(
+            slug="test123",
+            name="Test Exchange",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange)
+        db.session.flush()
+
+        # Register first user
+        participant1 = Participant(
+            exchange_id=exchange.id,
+            name="Alice",
+            email="alice@example.com",
+            reminder_enabled=True,
+        )
+        db.session.add(participant1)
+        db.session.commit()
+
+        # Register second user with different email (should succeed)
+        response = client.post(
+            "/exchange/test123/register",
+            data={
+                "name": "Bob",
+                "email": "bob@example.com",
+            },
+            follow_redirects=False,
+        )
+
+        # Should redirect to success page
+        assert response.status_code == 302
+        assert "/exchange/test123/register/success" in response.location
+
+        # Should have two participants
+        participants = db.session.query(Participant).all()
+        assert len(participants) == 2
+
+    def test_same_email_different_exchanges_allowed(self, client, db):
+        """Test that same email can register for different exchanges."""
+        future_close_date = datetime.utcnow() + timedelta(days=7)
+        future_exchange_date = datetime.utcnow() + timedelta(days=14)
+
+        # Create two exchanges
+        exchange1 = Exchange(
+            slug="exchange1",
+            name="Exchange 1",
+            budget="$30",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        exchange2 = Exchange(
+            slug="exchange2",
+            name="Exchange 2",
+            budget="$40",
+            max_participants=10,
+            registration_close_date=future_close_date,
+            exchange_date=future_exchange_date,
+            timezone="America/New_York",
+            state=Exchange.STATE_REGISTRATION_OPEN,
+        )
+        db.session.add(exchange1)
+        db.session.add(exchange2)
+        db.session.flush()
+
+        # Register for first exchange
+        participant1 = Participant(
+            exchange_id=exchange1.id,
+            name="John Doe",
+            email="john@example.com",
+            reminder_enabled=True,
+        )
+        db.session.add(participant1)
+        db.session.commit()
+
+        # Register same email for second exchange (should succeed)
+        response = client.post(
+            "/exchange/exchange2/register",
+            data={
+                "name": "John Doe",
+                "email": "john@example.com",
+            },
+            follow_redirects=False,
+        )
+
+        # Should redirect to success page
+        assert response.status_code == 302
+        assert "/exchange/exchange2/register/success" in response.location
+
+        # Should have two participants with same email but different exchanges
+        participants = (
+            db.session.query(Participant).filter_by(email="john@example.com").all()
+        )
+        assert len(participants) == 2
+        assert participants[0].exchange_id != participants[1].exchange_id