Here is Dup 2.0 beta 1 for Avisynth 2.5.
It supports both YV12 and YUY2.
The algorithm was changed so that when there is a string of duplicates the last frame of the string is used as the copy frame (previously the first frame was used).
In deference to the great sh0dan, the copy parameter was defaulted to true.
Feedback will be appreciated.