Minecraft wiki entry for Block_and_item_duplication, associated text: TNT duplication works in a similar way as rail duplication. A TNT block is BUD-powered, such that when the contraption is pushed by a piston, the TNT is updated and ignites into a primed TNT entity. Then, the piston moves the TNT from the list of TNT block left. This doesn't so much duplicate the TNT block as it creates free primed TNT entities from it without deleting it. Many TNT duplicators still work, but unless properly constructed will blow themselves up. TNT duplication is unreliable and will fail sporadically. This appears to have been patched in version 1.16.4 [verify]