How do you repeat missions in gta 4?

Answer 1

You can not repeat story missions and most of the side-missions in the game, unless you re-start the game, or re-load an old save file. However, there are some special events which you can repeat, such as the Gran Turismo race and the RC Car missions (blowing up Maffia Sentinels, Yakuza Stingers, Diablo Stallions). You can also enter some cars which will take you into a mission where you need to hit checkpoints in a short amount of time, and if you are unable to complete these missions, you can try again.

Answer 2

If you have saved the completed mission, you cannot repeat it.