What good are exit polls if voters vote many times?

Exit polls can be used to determining if vote fraud is occurring, but the exit polls fail if voters are being bused from poll to poll to vote many times. From the Canadian Press:

Prime Minister Vladimir Putin won Russia's presidential election on Sunday, according to exit polls cited by state television, but the vote was tainted by claims of violations, including "carousel voting" in which voters were bused around to cast several ballots.
Putin tallied 58 per cent of the vote, according to a nationwide exit poll conducted by the VTsIOM polling agency. Another exit poll done by the FOM polling agency showed Putin received 59 per cent of the ballot.
Official vote results from the far eastern regions where the count was already completed seemed to confirm the poll data. With just over 14 per cent of all precincts counted, Putin was leading the field with 62 per cent of the vote, the Central Election Commission said.
But if thousands of claims of violations made by independent observers and Putin's foes are confirmed, they could undermine the legitimacy of his victory and fuel protests. The opposition is gearing up for a massive rally in downtown Moscow on Monday.
"These elections are not free ... that's why we'll have protests tomorrow. We will not recognize the president as legitimate," said Mikhail Kasyanov, who was Putin's first prime minister before going into opposition.
Golos, Russia's leading independent elections watchdog, said it received numerous reports of "carousel voting," in which busloads of voters are driven around to cast ballots multiple times. . . .