Mac's Dupe Machine (aka Dupey) is located in Mac's old underground evil lair. However, it was shut down by the 1.7.3 update in Minecraft. Mac mostly made duplicates of rare items including diamond blocks, and TNT.
In Episode 171 - Danger! Dupe Exploit! Mac shows Zack Scott her Dupe Machine. Zack does not know what it is or what it does before she places a diamond block next to the machine and activates it. ZackScott replies with an extremely loud "No!"
In some of Zack's videos, Mac had gotten into trouble for giving out Diamond Blocks in the Jail or other inhabited areas.
In a recent video, Zack seen in an underground mine where blocks of diamond and gold were on the ground to which Zack said "What are they doing here?", which caused Mac to giggle and Zack to respond with "Come On!"From the Episode 400 map download, if you go down to Mac's Evil Lair you can see a sign outside of the Mac's Evil Laboratory saying "RIP Dupey".
The Dupe Machine's purpose was rendered obselete by the inclusion of Creative Mode.