I am facing same issue in Magento2, After the order gets confirmation we receiving the order confirmation mail twice. We using External plugin JccPaymentGateway , by using that plugin if we do order it sends one mail. But if we select inbuilt magento2 payment option like (Cash on Delivery, Cheque) option sends order confirmation mail twice.
Magento Version : 2.2.6
If you know the solution please reply as soon as possible