Is this the same issue reported here: http://forums.adobe.com/thread/1160457?tstart=0
If you don't have any transitions on the element you duplicate, please add more detail on how to reproduce this, or provide a sample file that shows the error.
Yes I know - I posted more info there.
So you do have transitions on these elements when you duplicate them?
I PMed the file.
Thanks Marie. Yes, this indeed looks like the same bug.
The workaround for now is to position the playhead at the start of the transition in the timline of the element you want to duplicate, select the element, and then do a Copy/Paste Special > Paste All...this will essentially duplicate the element with everything on it, and avoid the exception.