I too am using the Neat Desk scanner, with the Neat 5.0.25_43 software installed - and having duplex problems.
Windows 7 - 64bit. The scanner driver is Version 184.108.40.206 dated 6/18/2011 with Twain drivers:
windows\twain_32\Neat ADF Scanner\NeatADFScanner32.ds
windows\twain_64\Neat ADF Scanner\NeatADFScanner64.ds.
The code below returns: DuplexMode.none
DuplexMode mode = _device.DocumentFeeder.DuplexMode;
I've also tried device.Capabilities.Find(DeviceCapabilityId.Duplex) - and it returns null.
HOWEVER, if I activate the driver UI, and set duplex in the Neat UI (where it is exposed), then it works. Go figure.
On Windows 7, control panel devices and printers, when I select the Neat ADF Scanner, right click on "scan profiles", then edit the Neat profile, it shows no duplex option for the scanner either. When I right click for properties of the scanner/hardware/properties/Details, select dropdown and select capabilities, it shows:
To be honest, it appears the Neat Driver is not properly exposing the duplex capability, and only supporting it through their UI. Most annoying. Anyone found a solution?