When to consider a Dual RW is when the trailer is an ultra wide 101". Usually trailers that wide are also over the 16K GVWR and for stability purposes a Dually is a better choice. The trailers are wider than the SRW trucks.
Tomorrow I pick up a brand new 3660SUITE that will be towed with my 2012 F350 SB. According to the manufacturer, the UVW and pin weights, plus what we normally carried in our last trailer puts the trailer ~12628 pounds. @ 18% pin that gives me ~300 available payload. The empty pin weight is ~15.5%. Even if it jumps to 20% pin weight I am still OK, within ratings.
Here's the only thing I am a bit leery on, it is 42.5' long. I don't foresee issues, but would be more comfortable with a LB truck. The extra 18" of WB just gives it a bit more stability. I don't see it as Dually territory though. Wider, Heavier, and over 42' and then I would consider Dually.