1. Claim somewhere.
2. Place a block in the border of the claim.
3. Place what you want to dupe outside of the claim, close to the block in the border.
4. Tell to another player without permission in the claim place a Long-Range Breaker.
5. It's all, see the video for more info.

Video: https://www.youtube.com/watch?v=NvNTE3dG114
Reported too in: https://github.com/MinecraftPortCentral/GriefPrevention/issues/903

Direct support for sponge/plugins is not provided by AA.