Here it is,
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1. Get 2 Accounts Logged in.
2. Get Both accounts to the second trade screen (with the item your duping in)
3. The account without the item accept on the second screen.
4. Now the account with the item, you want to accept then quickly click decline
on the second trade screen
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I've never played a private server that didn't give you everything you needed, but if I ever feel the urge to play one, I'll be sure to give this a try.