Well, it's true in principle but not really in practice. Each time you dupe, you put the tones through another tone curve and you necessarily discard some tonal information. There is no perfectly linear tone curve in neg materials (although you can get quite linear, you can never get a perfectly linear curve unless you resort to that technology that we do not discuss here). Also, there can be the other technical issue of base fog... you add a bit every time you dupe. You might think, well I can just "print through" it, but still it cuts into the tones when you 'flip' the neg.
Granted, in many cases, what I am saying really doesn't matter because the details in the highlights and shadows on the toe/knee aren't so vital to a particular shot... and of course you can compensate for these effects in various ways. E.g. when neg duping, people often use different films and developers at each stage, aiming to retain as much tonal info through the whole process as possible while still ending up with a printable neg. There are numerous recipes for this and plenty of room for exploration, e.g. SLIMT might be a great way to keep highlight detail in the original neg and give it a better chance of showing up in the interpositive.
But... based on the abovementioned technicality, I stick to my answer: no, an analogue neg of a neg isn't a positive in the same sense that an original slide is a positive. The neg/neg process will have put the scene's tones through two curves, and maybe a third if you then print the positive. There will be generational loss.
Of course, there is a long history of successfully duping negs, e.g. Weston. I dabble in this and it can work well. But it turns out that the interpos you typically want for that process is not much like a 'real' slide, that'd be too contrasty unless you really compensate in development (I say this because I also turn slides into negs from time to time). The dupe neg is still good because the extreme tones can be picked up by the neg material; neg materials have miraculous ability to record extreme tones if you develop for it. We have to remember that neg duping works so well because the final destination is paper... and there's a big difference between that and projecting an interpositive.
I should add that I have 'printed' negs to glass slides and then result can be something quite close to a projectable slide, with quite good Dmax and Dmin. It is possible to get pretty good b&w slides that way.
