Customer got charged twice and has 2 subscriptions in Stripe

Debounce helps but isn't enough. Belt and braces:

1. Before creating a checkout session, check if the customer already has an active/trialing subscription. If yes, send them to the customer portal instead.
2. Reuse one Stripe customer per user. Store stripe_customer_id on your user and pass customer: to the session. Without it every checkout can create a new customer and you can't even detect duplicates.
3. Disable the button while the request is running (real disabled state, not debounce).
4. Make the webhook idempotent. Unique constraint on stripe_subscription_id and upsert. Also store processed event.ids:

   create table stripe_events (id text primary key, processed_at timestamptz default now());

   Insert first, and if it conflicts, skip. Stripe retries and can deliver an event more than once.

Refunding and cancelling in Stripe was right. Your webhook should then pick up customer.subscription.deleted and clean up the second row.