Common causes: Gateway webhook retries (server slow to respond, gateway re-sends), server timeout/browser retry loops, and caching/CDN (Cloudflare) replaying POST requests that should never be cached.
The Cash on Delivery isolation test: Enable COD payment. Place a test order. If the order duplicates with COD (no payment processing), the cause is site-level (plugin, caching, theme). If it doesn’t duplicate, the payment gateway communication is the issue.
The 100% coupon test: Same logic — removes payment processing from the equation entirely.
Narrowing questions: Do ALL orders duplicate or only certain payment methods? When did it start? Any plugin updates around that time? Is Cloudflare or a caching plugin present?