Creating a duplicate payment method does not fail as expected.

General information

SDK/Library version: 3.33.0
Environment: sandbox
Language, language version, and OS: PHP 7.1 on Mac OS with Homebrew

Issue description

When creating a new payment method using the same credit card number, expiry, cvv, and postal code as an already vaulted card for a different customer, the request does not fail despite passing the 'failOnDuplicatePaymentMethod' => TRUE option. In other words, the code snippet in the Braintree API reference

$result = $gateway->paymentMethod()->create([
    'customerId' => '12345',
    'paymentMethodNonce' => nonceFromTheClient,
    'options' => [
      'failOnDuplicatePaymentMethod' => true
    ]
]);

does not work as expected. This is despite failing as expected when creating a new customer with an already vaulted credit card.

Steps to reproduce

Create two customers using different test cards submitted using the Braintree Drop-in UI, with options similar to the following

$result = $this->braintreeApiService->getGateway()->customer()->create([
  'firstName' => $user->getAccountName(),
  'email' => $user->getEmail(),
  'paymentMethodNonce' => $nonce,
  'creditCard' => [
    'options' => [
      'verifyCard' => TRUE,
      'failOnDuplicatePaymentMethod' => TRUE,
    ],
  ],
]);

If you use a duplicate card for one of the customers, it will fail as expected.

Create a new payment method for one of the customers using the same card number, expiry, cvv, and postal code as for the other already vaulted customer, using options similar to

$payload = [
  'customerId' => $customer->id,
  'paymentMethodNonce' => $nonce,
  'options' => [
    'makeDefault' => TRUE,
    'failOnDuplicatePaymentMethod' => TRUE,
    'verifyCard' => TRUE,
  ],
];
$result = $this->braintreeApiService->getGateway()->paymentMethod()->create($payload);

The $result will be successful, even though expected behavior is for it to fail.

braintree / braintree_php