Difference between single rear wheel and dually rear wheel

There are many things that might be different between a SRW and a dually of the same brand.
For instance the rear axle, tires, wheels brakes, springs. While some of those things may come from the same parts bins, meaning the same part numbers, some others may not. On your 2500 the wheels are definitely different. Same for the tires. Both of those things alone contribute to the lower rating.
I do not own a newer GM product, thankfully. But i did have an earlier version with a GVWR sticker of 9200 pounds. That truck loaded ready to travel scaled at 7500 pounds. Hitching my fiver to it the scaled weight jumped to 10,500 pounds. Pulling and stopping were never an issue. Handling in the mountains was an issue. It felt at times like the trailer was pushing me. Switched to a dually pulling the same fiver and the handling is as different as night and day.
Personally there is no way i would consider pulling a large toy hauler, or any large trailer for that matter without my dually.

As said, DRW is rated higher in GVWR and sometimes GCWR, both good when towing heavy.
If you have a rear tire blow out with 4000lbs of pin weight, I would want 1 extra tire to help control the load until I got pulled over.
More stable in the sway department with high winds and when you get passed by a semi doing 75mph.

A dually has 4 wheels/tires on the back axle.

Obviously, a dually can handle more load; both on the pin and oftentimes behind the truck. 1 ton SRW trucks are commonly rated a couple of thousand lbs. less on the rear axle than a comparable DRW 1 ton truck. That is why we went from an F350 SRW to a dually.

Many here will also tell you they like the stability of the dually better. We had an F350 SRW and then bought an F450 dually. Pulling our Cyclone, I never had stability problems with either truck. The SRW truck pulled our big Cyclone through some wicked storms and it was as stable as what I have now. But that's just me.