I have used the 24 to 70 on my D7000 with no problem. You can take the photos and start with the first one with the one spot and as you look at every 25 photos or show and you can watch the number of spots grow. Maybe it is dust in the camera its self but I am sure it has something to do with the shutter, because this is 2 cameras acting the same way.
By the aw I have also rented the D 4, D 3x and the D800 using the same lens with no dust issues. So I think that pretty much elimates the lens leaking and pulling in dust.
I have only had to clean the d 90 a couple of times over the past 5 years or so And I never really took any special precautions changing the lenses, before I knew about sensor dust.
The D 600 has a faulty lot maybe from a certain manufacturing plant or shiyyter parts tha were not assembled correctly.
I am totally ticked off that Nikon won't acknowledge a problem, I understand #### happens but also you make it right when it does.