More than one payment processor instance of one class breaks IPN.

A payment extension provides a payment class (deriving from CRM_Core_Payment) identified by a payment method name.

On IPN requests, the following static method is always called:
CRM_Core_Payment::handleIPN(), which calls
CRM_Core_Payment::handlePaymentMethod().
The latter fetches the payment processor by civicrm_payment_processor_type.name.

If two payment processor instances exist in the database for the same processor type, then this will be ambiguous, and CiviCRM will likely load the wrong processor instance.
Result: The IPN fails.