Minecraft wiki entry for Block_and_item_duplication, associated text: One common TNT duplication design uses a minecart on a detector rail to power the slime block it is placed on, and moves a TNT block next to a coral fan underneath, the combination of the TNT and the coral duplicates the TNT whenever the whole contraption is moved forward with a piston. For an indepth tutorial of this one checkout Cheesysponges video https://youtu.be/3Izrj2m6Li4