I used the item dupe cheat, located here: http://www.psypokes.com/forums/viewtopic.php?t=9518
on my Masterball and Rare Candy, I didn't get anything, in fact, I lost both items! Is there any way to get my Masterball back without restarting the game? I know at least one person has some good cheats.

There is the Lilycove lottery, but that's extremely difficult to win. Other than that and cheating, no, there is no way to get your Master Ball back.

thanks, and Kyogre, can I get that back after I caught it then lost it too? I could do that "restart my game" thing, but I still need to import my pokemon...

Nope, anything you lose in any kind of duplication is gone forever. (Gold, Silver, Crystal, Emerald, etc.)

duplication is risky, the people who make the games don't want you to do it so you could trade a pokemon for one or something, if you have another game with compatibility you could play that then trade the master ball over, that's how i have 6 masterballs on my Ruby.

Any duplicated items that are lost are lost for good. although you can check out the lottery. its extremely hard for sure good luck on the lotto