The ballot that will be handed to Illinois voters on November 2nd will cover two elections—special and regular—for the U.S. Senate.

Voters will be instructed to choose among Republican Mark Kirk, Democrat Alexi Giannoulias, Green Party candidate LeAlan Jones, and Libertarian Mike Labno in a special election for the lame-duck period extending until January 3rd, when all U.S. Senators are sworn in. Then voters will be asked to choose between the same candidates for the full six-year term.

The unusual double vote is the result of a federal lawsuit filed by voters who were unhappy that Rod Blagojevich appointed a senate replacement for Barack Obama (Roland Burris), rather than holding a special election to fill the seat after Obama was elected President.

I asked Chicago attorney (and former 43rd Ward alderman) Marty Oberman, one of the lawyers who brought the suit, if there’s a chance that a candidate could win the short term but lose the full six-year term.

He acknowledged the possibility, but called it “remote,” expressing confidence that the same man will win both races. Having studied previous, similar situations, he told me, there is typically a one percent spread—that is, people will almost always vote for the same candidate.

But what if the margin is razor-thin, and two different winners emerge? After all, this midterm election is sui generis in many respects—including widespread handicapping of what might happen in the lame-duck session that starts later next month. Could that lead to the situation where someone is senator for just over a month and then has to give the seat the winner of the full term?

Oberman offers a hypothetical of a conservative Democrat who selects Kirk for the short term because he worries that Obama, almost certain to face more Republicans in the new Congress, will try to shove legislation through the lame-duck session. This voter, Oberman explains, fears that rush of legislation. But, Oberman adds, the voter believes in the long-term Democratic agenda, so he votes for Giannoulias for the full term.

More likely, though, is the following scenario: Many voters will go to their polling places expecting to select a senator. Asked to vote twice (and unaware of this year’s peculiar situation), they’ll be confused and suspicious, perhaps even suspecting they’re witnessing an extreme example of that old Chicago slogan, “Vote early and often.” More important, they’ll figure that their double-dipped ballot would be invalidated—and vote only once.