From the NY Post:
Records show at least 153 New Yorkers actively registered in both New York City and at their upstate homes voted in the 20th Congressional District's special election in March, 76 percent of whom were enrolled Democrats, according to elections records obtained by The Associated Press. Nearly 250 more in the district are actively registered upstate and down, but didn't vote in that particular election.
It's illegal to be registered in two places at once, but the state Board of Elections said it probably happens because New York City boroughs are behind on eliminating voters from the city database after they switch their registration.
The board said people are legally required to vote from their primary residence, but that's not clearly defined.