Since the duplicate reminders are sent at exactly the same time, I would try to separate two possibilities:
1. The reminder job is being triggered twice.
2. The reminder job is triggered once, but the SMTP layer sends the same message twice.
A few low-risk checks may help narrow that down:
1. Check whether both duplicate emails have the same subject, recipient, timestamp, and message ID. If your SMTP plugin logs outgoing emails, compare the two log rows. If there are two different SMTP log rows, the duplicate likely happens before or inside WordPress. If there is one SMTP log row but the customer receives two emails, the issue may be downstream from WordPress.
2. Check WooCommerce > Status > Scheduled Actions and search for CusRev/review reminder related actions around the exact time the duplicate emails were sent. If there are two matching scheduled actions with the same order/customer/time, that points to duplicated scheduling.
3. In the Customer Reviews settings, confirm there is only one active reminder rule/campaign for the same order status and timing. Also check whether both an automatic reminder and a separate follow-up/reminder option could match the same completed order.
4. If possible, test with one internal test order and one internal email address. Move the order through the same status path once, then record whether one or two reminder actions are created before the email is actually sent.
5. Since you mentioned WP SMTP, I would not assume it is the cause yet, but I would keep its email log enabled during the test. The key evidence is whether WordPress creates two outgoing email events or only one.
If you can share a redacted screenshot of the relevant reminder settings and the matching Scheduled Actions rows around the duplicate send time, it should be easier to tell whether this is duplicated scheduling, duplicated order-status triggering, or duplicated SMTP sending.
