The diodes in the module are only to COM. I would never rely on the diodes built into the module anyway. Page 7 of manual for the module http://www.ni.com/pdf/manuals/323355b.pdf calls out for external flyback diodes that are installed at the inductive load.
For a flyback diode to function correctly it must be installed in close physical proximity to the inductive load. Otherwise the inductance of the wire between the diode and the coil causes problems of its own. One of which is inducing electrical noise and hash in adjacent wiring.
