Hi,
Depends on what you're trying to do, but generally,
no, you don't need Automatic Duck.
OMF is currently "one-directional" in an FCP work flow:
FCP->OMF->PT->mixed aif->FCP
FCP will export an OMF
DigiTranslator makes that OMF into a PT "session"
Output the resultant mixes as .aif files
import those .aif's into your FCP project and place them in the timeline.
Output to deliver format.
If that's not what you need to do, then all bets are off!
Good luck.
tc