Check for existing completed payments in createPayment to avoid duplicate payments with onNotify

Problem/Motivation

We are having duplicate -Direct 2c2p- payments and we're suspecting a race condition that leads to having duplicate payments

The race condition we're suspecting is between onNotify and createPayment

Steps to reproduce

Create a delay before saving the payment inside create payment and let onNotify creates the payment and complete it
This will lead to a duplicate payments with completed state

Proposed resolution

Add a check inside createPayment for existing completed payments and prevent save if it exists