Is Duet compatible with Mac OS X Mavericks (10.9)?

Yes!

Duet FireWire, Duet 2, and Duet for iPad/Mac are all fully compatible with Mac OS X Mavericks (10.9 to 10.9.5).

Please be sure to download and install the latest Mavericks-compatible software package for your product from Apogee's site. 

Apogee Software Downloads