I have used it very successfully to prevent growth of grasses while planting sunflowers. I used it as both a preplant (sprayed and then incorporated) as well as pre-emergent, applied directly after planting. Yes it does have a residual, and that is the purpose and how it does so well.
The grass control in my sunflower patch was so good and so complete, that I didn't even spray Clethodim this year as I thought it would be wasteful; there just weren't any grasses to kill. Still had the usual broadleaf weeds, but I added Spartan pre-emergent and had fewer.
So if grasses had been a big problem in the past, Dual might be a good pre-emergent to try. Would love to hear other's input on the use for brassicas.
It's been a long time since I used any metolachlor (Dual). I guess it hasn't changed much. It and glyphosate are two entirely different animals in terms of how they work and what they control. As we all know, glyphosate must be applied to actively growing plant tissue for it to do its work. It has no soil residual activity.
Metolachlor (Dual) works much differently. It's a soil base herbicide. Think of it as a net that "catches" some grasses and weeds before they get going while allowing the desirable plants to grow thru the mesh. Dual stops seed germination. It must be incorporated INTO the soil. A good rain will do the job. It's good because it does have a residual. It stops weed seed germination for some period of time. I can't remember how long. I'd guess 10-12 weeks or a typical growing season. I think it requires more consideration and a good read of the label for best and safest use.