This is only possible on Servers when other players claimed chunks.

On the claimed chunk you have to enable block placement but not block breaking.

After that just place for example a cooling box. It will dissapear from your hand, but you are still able to place it more than once thus duping the block.
This will not copy chest inventory or any other sort of that, no Item dupe just blocks

with which claim system?

FTB Claims, none of the others which are not included...

There's an open PR on their github to fix this: FTBTeam/FTB-Chunks#213

Just waiting for it to get merged :)