chrisk123999: Found a duplication exploit in multiplayer. When one person leaves the room then breaks it the box, the player left inside can still pick up stuff and then leave. Duplicating anything that was there.

Tested on that latest version of Cauldron.

I can not reproduce this and require more information - I tested without cauldron though - are you running something you suspect of interfering?
I tried: Player 1 is inside the CM and dropped one piece of cobblestone on the floor, placed a chest and put a piece of dirt in it.
Player 2 is on the outside and breaks the blocks. All items (cobble, dirt and the chest) dropped on the outside and player 2 can pick them up. Player 1 is in an empty room, no items anywhere, no duplication occurred.
Can you be more specific on how this can be reproduced?

After a bit more testing it seems that it's not duping the items, but simply only removing the first 3x3x3 blocks in the room.

Noticed this at the bottom of my log during server shutdown:

[12:02:40] [Server thread/INFO]: Applying holder lookups
[12:02:40] [Server thread/WARN]: Unable to lookup CompactMachines:personalShrinkingDevice for public static org.dave.CompactMachines.item.ItemPersonalShrinkingDevice org.dave.CompactMachines.init.ModItems.personalShrinkingDevice. Is there something wrong with the registry?
[12:02:40] [Server thread/WARN]: Unable to lookup CompactMachines:atomShrinker for public static org.dave.CompactMachines.item.ItemAtomShrinker org.dave.CompactMachines.init.ModItems.atomShrinker. Is there something wrong with the registry?
[12:02:40] [Server thread/WARN]: Unable to lookup CompactMachines:atomEnlarger for public static org.dave.CompactMachines.item.ItemAtomEnlarger org.dave.CompactMachines.init.ModItems.atomEnlarger. Is there something wrong with the registry?
[12:02:40] [Server thread/WARN]: Unable to lookup CompactMachines:interfaceItem for public static org.dave.CompactMachines.item.ItemInterface org.dave.CompactMachines.init.ModItems.interfaceItem. Is there something wrong with the registry?
[12:02:40] [Server thread/WARN]: Unable to lookup CompactMachines:quantumEntangler for public static org.dave.CompactMachines.item.ItemEntangler org.dave.CompactMachines.init.ModItems.quantumEntangler. Is there something wrong with the registry?
[12:02:40] [Server thread/WARN]: Unable to lookup CompactMachines:interfaceblock for public static org.dave.CompactMachines.block.BlockCM org.dave.CompactMachines.init.ModBlocks.interfaceblock. Is there something wrong with the registry?
[12:02:40] [Server thread/INFO]: Holder lookups applied