This is a re-creation of Max's guide on Hard Duping, but made specifically for mac people.
The Hard dupe is an exploit used to obtain 2 of an item. doing this multiple times will give you even more of that item. This is useful in categories such as Journey runs, in which one can research Boss Bags and earn a specified item from them.
Executing the dupe: First of all, you need to open up your world and character in either multiplayer or singleplayer, put the items that you are wanting to dupe into your inventory, and then leave the world. Afterwords, you need to enter the world with that same character, but in multiplayer. Put the items that are in your inventory that you want to dupe into a chest, preferably empty, and then execute the following command: Cmd+Q (Alt+F4 will not work). This should close the game without logging out of the world. Once you enter the world again, you will see the items back in your inventory, as well as in the chest.
(Optional) Using multiple instances and a Local Server: Although not necessary, using this method makes the process even faster. You will need to create a Local Server and be able to open multiple Instances of the game.
Creating a Server: First of all, you need to have a terraria server running on your computer. Unless you go through the extra steps to, others may not join your world. You can download the server file at https://terraria.gamepedia.com/Server#Downloads. Nake sure you get the latest version of terraria. After you download the file, unzip it, open the unzipped file, click on mac, and store the "Terraria Server.app" file wherever you would like. Then, open up the file. If it says you can not open it because it is from an unidentified developer, got to System preferences, Security & Privacy, General, and look towards the bottom. Click on the lock in the bottom left and enter your username and password for your computer's main Admin profile. The go to the section labeled "Allow apps downloaded from:" and make sure "Anywhere is checked". You should now be able to open "Terraria Server.app". A prompt cautioning you that the file comes from an unidentified developer, and click "Open". Wait for a little until a list of all the worlds that you have on your computer come up, and identify which number the world that you want to dupe on has, and type that in and enter. After that there is a series of prompts that you need to type in. Answer accordigly: "1, (press enter), n, (press enter)", A slew of numbers should come up and then cleared with the message "Server started" coming up afterwords.
Joining the server: You may now go in-game, and click on the Multiplayer section, then click on "Join via IP". Choose character. Then type in "localhost" and press enter, then pressing it a second time. If you are not connecting immediately, then make sure that both the world and the character are either Journey mode or Non-Journey.
Opening up multiple instances: To open up multiple instances of Terraria, open up terminal, located in the utilities folder in Applications. Run the following command in the Terminal: "open -n /Users/admin/Library/Application\ Support/Steam/steamapps/common/Terraria/Terraria.app" This should open terraria. You can run this multiple times to have even more instances of terraria.
Purpose of using Multiple instances and a server: If you use multiple instances and a server, you can just quit out of one terraria, and join with another, not having to boot the game up again, greatly diminishing the amount of time it takes to rejoin
