I agree with Daniel. However, that is likely only going to be the case with bank properties and motivated/distressed sellers. Sellers who have a vacancy during this time of year become more motivated for sure! That being said, as we turn the calendar and move into January, prices are generally strong. I've been working in this area for over a decade and live 15 minutes from Green Bay. As an investor, I like to pick up properties from motivated sellers in the month of December, but as a Realtor, I find my biggest month every year is February! I find that in our area, the slowest months for sales (and deeper discounts) are in the summer when all the buyers are out playing and enjoying vacation while their kids are off school. The buyers come out strong and buy everything up in late January thru end of March (believe it or not...I have years of numbers to prove it!). BUT, buying a duplex is a bit different since the motivation of the seller is a lot different than an owner/occupant of a single family home...if they are vacant in the winter, and they're a tired landlord, they will just want to get it gone.
All that being said, if you don't like the property you have your eye on, it's ok to wait for the right one. After all, you'll be living there. If the numbers work, and you're comfortable calling it home, then make your offer and see what you can get it for. Don't just buy it because of the season...you said you're not in a hurry. If you can wait and watch the spring market for a better fit, you'll likely be able to secure a nice deal. Everyone thinks listing in the spring is the way to go, but I find every year that the best time to actually sell is the first quarter. By the time Memorial Day rolls around, most buyers have already purchased something and are off in their vacation mode.
My 2 cents...for what it's worth.