The closest Congressional race in the nation could be decided by people who don't really live in the district, and a Fox News investigation has uncovered evidence of voter registration illegalities, including one voter who apparently voted twice.

The candidates, Long Island Democratic incumbent Congressman Tim Bishop and Republican challenger Randy Altschuler, have been as close as just 15 votes apart, out of roughly 180,000 cast in New York's 1st Congressional District. The latest count puts Democrat Bishop ahead by about 235 votes. But both sides are now challenging roughly 2,000 absentee ballots that will eventually determine if the seat stays Democratic or falls to the Republicans.

A Fox News Voter Fraud Unit review of approximately 438 of the absentee ballot voters, who also maintain mailing addresses in New York City, reveals that 48 have double registrations. They voted by absentee ballot in Suffolk County, but are also listed as current "active" voters on the New York City rolls. Being registered in two separate jurisdictions is illegal and is a felony in New York State.

In addition, our investigation reveals that one absentee ballot was apparently submitted in the name of a Democratic voter enrolled in Suffolk County, while election records at the Board of Elections in New York City show that the same voter voted, on Election Day, in Manhattan.

"It certainly is illegal to vote twice," warns John Conklin, the spokesperson of the New York State Board of Elections, who said convictions can bring a one year jail sentence.

"If you voted twice, you committed a felony, and I think it would be very difficult to do that by accident. I think that would be something to be concerned about and that the district attorney is going to come knocking on their door."

The voter in question has an extensive voting record, having cast a ballot in New York City 33 times since 1984. Records show he not only voted in a Manhattan polling place during the general election this past November, but also in the Democratic primary in September. The voter has not returned our request for comment.

The overwhelming majority of the Long Island voters, who are also on the New York City voter rolls, appear to be wealthy Manhattanites who own second-homes in the posh resort towns of East Hampton, Southampton, Montauk, Amagansett and Shelter Island. Many of them voted in Manhattan as recently as last November in New York City's mayoral election, and in the presidential election of 2008.

It is not illegal for second home owners to vote from their vacation homes, as long as they are not also registered or vote elsewhere. In fact, the Democratic Party has encouraged the practice.

The website, Countryvote.org, which is run by the New York Democratic Lawyers Council, says that "New Yorkers have the right to choose where they vote if they have more than one home...you have the right to vote using an absentee ballot if your work or family commitments keep you in the City during the week."

But Countryvote.org does say that it "can't give individualized legal advice."

"They are abusing the system," charges local resident Noel Feustel, a long time voting activist who has tried to bring light to voter registration problems.

"They are disenfranchising the locals, and negating my vote. They are affluent enough to own multiple residences, but the regular person, the poor guy harvesting scallops doesn't have that option. It is one person one vote, not multiple residences, multiple votes. If they want to claim that this is where their heart is and vote here, they should serve jury duty, pay sales tax on their BMW's and Steiger Craft boats, and when they move on to their final domicile, pay estate taxes in Suffolk County."

On Tuesday, lawyers for both candidates will be in court on the absentee ballot challenges.

The Fox News Voter Fraud unit investigates voter issues across the country, and you can reach Senior Correspondent Eric Shawn and Producer Meredith Orban, at Voterfraud@FoxNews.com if you suspect voter problems where you live.