The usual approaches involve using a commercial rubber-renewal product to restore the original elasticity (as much as possible), plus a light touch with fine sandpaper while the wheel is turning, to restore roundness and grip.
If you're in a hurry and can't find a product that renews rubber, you might try detergent or glycerine on the wheel; both are good for rubber. Meguire's rubber and vinyl cleaner and restorer (from auto-parts stores) is good, too. NEVER put any kind of oil on a rubber item; oil will chemically ruin most rubber products in a hurry. Be careful when lubricating the wheel's bearings, to not let any of the lubricant get on the rubber part of the wheel, not even a little bit from a drip or a careless finger.
When sanding, take off JUST enough to remove the outermost layer of hardened rubber; you don't really want to alter the diameter, just give the surface back a little "tooth" so it will "grip" well. You must do this only when the wheel is spinning fast, so you keep the roundness (circle shape) of it. Less is usually better, when it comes to sanding, and try to keep the original shape/contour of the wheel; you may need to angle the sandpaper block a bit on each side.
Also, remember that the other side of the equation is the surface that the idler wheel rubs against: Make sure it is absolutely clean, and free from any oils or waxes or lubricants. You may use something like 99% alcohol or lighter fluid or acetone to clean it; these will remove almost everything, and any residues will evaporate off completely. Make sure you give them time to fully evaporate, before letting the idler wheel come back into contact with them.
If that isn't sufficient, then you can send the wheel out to any of several people online who make them like new again, by completely replacing the rubber portion.
Sometimes the problem isn't just the rubber part, but also the bearing. Try cleaning and lubricating the bearing, and if it still isn't good, but the shaft is good, then look for another wheel from a donor unit. [It is usually best to keep wheel and shaft together, but you can replace either one independently.]
For Lencos, you can buy after-market precision idler wheels made from titanium off of eBay, and even non-resonant support rods (shafts) for them, but I'm not sure if anything like that is available for Duals. A professionally-restored (new rubber) original one should come close.