1.Go to the battle tower at the battle frontier
2.Then deposit the pokemon u want to clone in the PC
3.Then exit the PC and save the game
4.Once you have saved the game take that pokemon out of the PC
5.Enter that pokemon an another in the double battle thing
6.When the lady says would you like to save say yes
7.When it says saving dont turn off the power turn the game off
8.Turn the game back on you will have that pokemon in the PC and your party

Look I know a guy said turning your game off like that wrecks the game, but it does'nt trust me

THIS ONLY WORKS FOR GBA

Okay, first go to the battle frontier. Then go into the Battle Tower. Go to the PC on the far right side. Have the pokemon you want to copy in your party. Then go to the PC and Deposit the pokemon you want to copy. Turn the PC off. Make sure you only have I pokemon in your party. Stand in front of the PC and save the game. Then withdraw the pokemon you want to copy. Next go to the lady closest to teh PC. Choose Open Level. She will ask if she may save the game. Say yes. When the overwriting of your data page appers, turn your game off and back on. Go to the PC and WALLA!! The pokemon you wanted to copy is in the the same spot you left it before.

if u want clone pokemon go to the battle tower {battle fronteier} make sure no pokemon r in box 9 u can only do 4 put them in there andlogout of the pc save and then withdraw them go to the lady next to u and start the battle make sure it is open level when it says saving do not turn power off TURN OFF FAST!!!! then u got ur party full check box 9 and u have cloned pokemon.
ps. make 1 of ur pokemon hold the master ball then u got 2!

if you want to more clear awnsers go yo youtube and search for a video on "pokemon emerald cloning glitch"