If you're syncing OmniFocus with iCal on two machines and syncing iCal between the machines, that's probably the problem. Your best option is probably to sync to different calendars on your home and work machines. There's more discussion of this in a few other threads:
http://forums.omnigroup.com/showthread.php?t=6596
http://forums.omnigroup.com/showthre...0117#post30117