At one and the other Magento store we had the problem, that often (not always) two bills / double accounting for one and the same order created, if these bear PayPal (or. PayPal More) was paid.
This was of course uncomfortable, as some customers were confused when they received two bills, wondering, if they had ordered twice.
After a long analysis and debugging turned out, that (probably) by changing the payment method of PayPal to PayPal Plus another webhook has been deposited with PayPal, so that two confirmations were sent.
Where do you find these webhook? You register with them http://developer.paypal.com and goes on the Dashboard “MyApps & Credentials”. There you click on “REST API apps” to the relevant app. Then is important to distinguish between sandbox and live (top right of the app name). Below you will find then the webhook. If one on here, that for the same store two (or) are more webhook, the superfluous should be deleted. Generally provides a web hook URL from Sun: https://www.dies-ist-meine-domain.com/index.php/paypalplus/index/webhooks/.
After the cancellation of the second webhook there were only a statement and accordingly even a mail to the customer with the bill.
Used in Magento Version 1.9 Comments? Additions? Notes? Gladly!