As you can see from the picture that they look very similar, specially as Juv. The big difference is around the tail as the Lavander tang have 2 dots just before the tail which the Dussumieri doesn't. Also on the face the Lavander tang has dot while the Dussumieri doesn't have them and they may have the start of the yellow band by the eye. The Dussumieri Juv for the most part are no where as colorful as the adult, specially in the body and the tail, so small ones look very similar to Lavender tangs.
FishTown gets them in, usually a whole bunch of them, and either labels them as Lavander or Palani tangs. It's very possible that a couple got mixed in with a bigger order, I found the one I got like that, but if they where truely shipped as Dussmieri tangs they would start around $100 each.
They do get very large and need a good size tank. On top of the Juv that I have I also have a very large one, 12"-14" range which I've had for about 8-10 years.
They also get in blochii tang which look very similar to the Dussmieri, they even have the Yellow band by the eye. And they go for a lot less than the Dussmieri.