Two Places:
http://www.serebii.net/pokearth/sinnoh/distortionworld.shtml
http://www.serebii.net/pokearth/sinnoh/turnbackcave.shtml
When Battling Giratina in the Distortion World, It's OK if you Kill it. Giratina WILL Re-appear in TurnBack Cave. I do Not know if It'll have the Same Battle Music, but both Times, He appears at Lv.47 and knows the same Moves. However, Giratina will appear in Altered Form, as it's the real world instead of the Distortion World. But That's OK, It is over the Portal, so just get the Griseous Orb.