Attempt to get ftp://ftptest@qt-test-server/...
or ftp://ftptest:wrongpassword@qt-test-server/...

Login fails (the first case uses "anonymous@" as the password, which is a separate problem)

QFtpPI emits the error signal, which triggers QNAM to emit authenticationRequired via QNetworkAccessFtpBackend::ftpDone()
The user code supplied the correct credentials (ftptest:password)
This queues a login on QFtp
However QFtpPI has an empty command queue, so the finished signal is emitted - which calls QNetworkAccessFtpBackend::ftpDone() again. This emits authenticationRequired a second time.

Test case - tests/manual/qnetworkaccessmanager/qget
By design, it only supplies login credentials once for each request.