This dupe method has been patched for months, but since it's patched i guess i can say what that turk probably said in those pms
You need 1 NCS but it works alot better if you have 2 and its pretty simple. pick a name that you know no one else will have, but don't name change yet. If you have 2 NCS send one of them to the name you picked out along with any other items you wish to dupe but there is a limit of like 10 or something. After you sent all the items to your predicted name. Then use the other NCS to change into that name, you should have 2 of each item you sent in the mail. If you sent more than 5 items then it won't work because there is a limit to how many letters you recieve at one time. This is where using 2 rather than 1 NCS comes in, if you used 2 you can continue doing this all day, if you used one this is as far as you go.
But anyway, its patched