diff --git a/fjccv_webhook.py b/fjccv_webhook.py
index 2ecca3e..e8cd338 100644
--- a/fjccv_webhook.py
+++ b/fjccv_webhook.py
@@ -30,6 +30,46 @@ from update_ledger import add_donation as ledger_add_donation
 STRIPE_SECRET_KEY = os.environ.get('STRIPE_SECRET_KEY', '')
 WEBHOOK_SECRET = os.environ.get('STRIPE_WEBHOOK_SECRET', '')  # whsec_... when Stripe is configured
 OUTPUT_DIR = os.environ.get('FJCCV_RECEIPTS_DIR', '/root/fjccv-receipts')
+PROCESSED_EVENTS_FILE = os.environ.get('PROCESSED_EVENTS_FILE', '/root/fjccv-receipts/processed_events.json')
+
+# Thread-safe lock for file operations
+import threading
+processed_events_lock = threading.Lock()
+
+def is_event_processed(event_id: str) -> bool:
+    """Check if a Stripe event has already been processed."""
+    if not event_id:
+        return False
+    try:
+        with processed_events_lock:
+            if not os.path.exists(PROCESSED_EVENTS_FILE):
+                return False
+            with open(PROCESSED_EVENTS_FILE, 'r') as f:
+                processed = json.load(f)
+            return event_id in processed
+    except (json.JSONDecodeError, IOError) as e:
+        app.logger.warning(f"Error reading processed events file: {e}")
+        return False
+
+def mark_event_processed(event_id: str) -> None:
+    """Mark a Stripe event as processed."""
+    if not event_id:
+        return
+    try:
+        with processed_events_lock:
+            processed = set()
+            if os.path.exists(PROCESSED_EVENTS_FILE):
+                try:
+                    with open(PROCESSED_EVENTS_FILE, 'r') as f:
+                        processed = set(json.load(f))
+                except (json.JSONDecodeError, IOError):
+                    pass
+            processed.add(event_id)
+            os.makedirs(os.path.dirname(PROCESSED_EVENTS_FILE), exist_ok=True)
+            with open(PROCESSED_EVENTS_FILE, 'w') as f:
+                json.dump(list(processed), f)
+    except IOError as e:
+        app.logger.warning(f"Error writing processed events file: {e}")
 
 # Sami's PC for receipt delivery
 SAMIPC_HOST = os.environ.get('SAMIPC_HOST', 'hello@100.85.236.10')
@@ -287,7 +327,13 @@ def stripe_webhook():
         abort(400, 'Invalid JSON')
     
     event_type = event.get('type', '')
-    app.logger.info(f"Received event: {event_type}")
+    event_id = event.get('id', '')
+    app.logger.info(f"Received event: {event_type} (id: {event_id})")
+    
+    # Check for duplicate events
+    if event_id and is_event_processed(event_id):
+        app.logger.info(f"Duplicate event skipped: {event_id}")
+        return jsonify({'received': True, 'status': 'duplicate'})
     
     # Handle checkout.session.completed (Stripe Checkout Payment Links)
     if event_type == 'checkout.session.completed':
@@ -322,7 +368,7 @@ def stripe_webhook():
         # This is important because Stripe requires a quick 2xx response
         thread = threading.Thread(
             target=_process_checkout_session,
-            args=(session, donor_name, donor_email, amount, transaction_id, date_str, year)
+            args=(session, event_id, donor_name, donor_email, amount, transaction_id, date_str, year)
         )
         thread.start()
         
@@ -332,9 +378,13 @@ def stripe_webhook():
     return jsonify({'received': True})
 
 
-def _process_checkout_session(session, donor_name, donor_email, amount, transaction_id, date_str, year):
+def _process_checkout_session(session, event_id, donor_name, donor_email, amount, transaction_id, date_str, year):
     """Background processing for checkout.session.completed events."""
     try:
+        # Mark event as processed immediately to prevent duplicate processing
+        if event_id:
+            mark_event_processed(event_id)
+        
         app.logger.info(f"Processing donation: {donor_name}, ${amount:.2f}, {date_str}, tx: {transaction_id}")
         
         # Get donation number