Pole-sitter Kubica, who last month became the first driver to take a second series win, had looked to have blown his chances of the double victory - and fourth 2005 win - when he opted to stay out on a dry track when it started raining.
Local driver Markus Winkelhock had gone into the pits to change for wet tyres earlier than Kubica and had inherited the lead when everyone had made their mandatory pitstops. However, Winkelhock made a mistake and ran off the track, handing back Kubica the lead and leaving him unchallenged to the flag.
Like the first race, Tristan Gommendy finished second, again achieving the position despite going off track because of mistake. Winkelhock finished third.
Jaap Van Lagen had his best race since finishing second at Monaco with fourth, having taken the position from Carlos Miguez with two laps to go. American Colin Fleming put in a storming drive from 20th on the grid to finish sixth.
Kubica's main Championship rival Will Power had another nightmare race, after spinning to avoid a collision with Gommendy. The Australian finished 14th.
The race finished two laps early, after the 45-minute allotted time for the race expired.
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