Duplication bug involving pistons and item frames

Watch this video for more detail.at 1m44.

— VIDEO —

To reproduce it's very simple.

1. Place a piston
2. place a item frame behind the piston.
3. place item you want duplicated in item frame.
4. activated the piston.
5. recover item of item frame at the moment when the piston returns.

Personally I tried and it works.
Thank you for correcting this bug.