in platinum no but in diamond n pearl you can
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In Pokemon Diamond, Pearl, and Platinum, you cannot 'recatch' Dialga, but you can have multiple attempts if you save before battling Dialga. In Pokemon HeartGold and SoulSilver, when you do the Arceus event, you automatically obtain your choice of Dialga, Palkia, or Giratina.
In Pokemon Diamond, you get Dialga before the 8th gym battle. After you beat the 7th gym, you battle at the peak of Mt. Cornet, Spear Tower, and battle, preferably catch Dialga, then you can access Sunyshore City. In pearl you battle Palkia.
No one really knows, it's nature can be different with each game that's being played. Palkia's faster, but Dialga can learn trick room. Regarding trick room, Palkia can learn earth power as a comeback, which has 90 power and 100 accuracy and is super effective against Dialga, but Dialga knows roar of time, which has 150 power and 90 accuracy. I've tried this before and used it on a Palkia, Giratina, and Arceus. For Palkia, it sucked against roar of time. It was a one hit KO.
You have to have arceus first in your team. Go to the ruins beside city and Cynthia should talk to you about arceus. You can now go into one of the rooms that was locked before. You will see an event to do with arceus and Cynthia and three pokeballs will appear. In them are palkia dialga and giratina. You can only choose one. You have to have an empty slot in your party for this to happen in the first place though.If you have an arceus then you can go to the ruins and trigger this trippy event where you see the world forming and stuff and then it gives you the option of taking a lvl 1 dialga, palkia or giratina
before you go to sunnyshore you need to get the main legendary in your game, pearl=palkia,diamond=dialga,platinum=giratina, once you've gotten it try to go there now and the guy will let you pass