I've seen some posts from some years ago about this, and I wanted to write about my solution to an issue I was having for anyone searching this problem now or in the future. In my particular case, having a mixture of mono tracks with stereo tracks in my Avid timeline caused Automatic Duck FCP Import plug-in to mis-translate the sequence into FCP7, resulting in an improper audio import.
It's not the fact that the Avid timeline has stereo and mono tracks together in the same edit, it's that mono clips & stereo clips are in the same AAF export running through the Duck Import FCP. I had other sequences that had one stereo track at the bottom of the Avid edit but no clips on that track, and there was no error importing that AAF into FCP via Duck.
With only mono tracks exported and only stereo tracks exported as separate AAF's, FCP Duck Import will work ok. The only difference is that the stereo tracks will import into FCP as a nested clip on one track. When you double-click that nested clip in FCP7, it opens a separate sequence and shows you the stereo track on two audio tracks. Hope this saves someone a bunch of frustration someday.