The printer does not support automatic duplex printing (double-sided printing). The duplex printing unit is only available on the MF655Cdw and MF657Cdw models, but not on the MF651Cw. Only single (one-sided) printing is possible on the MF651Cw.
The Canon i-SENSYS MF651Cw printer is compatible with Mac OS. You can find the applicable drivers for Mac OS on the support page under the "Drivers" section.
We have investigated as this device does not have a duplex function. Apparently the error was already adjusted once and reinstated at the beginning of the month due to a system error.
We will have it removed again. Thank you for letting us know.
Yes, it is possible to scan directly onto the platen directly onto a USB stick, or from the PC/Mac and even from the Canon PRINT app (iPhone/iPad but I tried that a long time ago).