You're not seeing double, and the two "United States Senator" listings at the top of your ballot, with the same contestants, is no misprint.
You vote first for a Senator who will serve from January 2011 through January 2017. Then you pick a Senator to speak for you through the bleak dark days of December, to the solstice on December 21, and a bit beyond.
The second Senator election is to fill Barack Obama's term, which expires January 2. It's a term with a novel history, Obama's ascension having made it f'ing golden, Roland Burris filling it since January 2009, then a federal court requiring today's vote to decide who will wrap things up. The choices in both elections are Mark Kirk, Alexi Giannoulias, LeAlan Jones, and Mike Labno. The winner of the expiring term will replace Burris as soon as the state board of elections certifies the results, which it expects to do by late November.