You can make a dupe neg on Kodak's 6121 Duplicating film. I have made literally hundreds of duplicate B&W and colour negatives over the past 25 years using this method.(posted 8205 days ago)6121is a process E-6 film, so you will either have to send it out, or try and process it yourself. I would consider the former, but not the latter.
I have also used the two-step method described above, and it works well. The main problem is dust, which you will encounter at both the interpositive-making and the internegative-making stages. Dust on the interpositive will print white in the final print, dust on the internegative will print black.
I used to use TMAX-100 for the positive, especially when the original was a colour negative, as it wold maintain the correct tonal relationships of the original, and not give you an "ortho" or "colour-blind" look in your final print. You can still get this effect if you want, by use of filters when making the interpositive.
I used to use Kodak Pro Copy Film for the negatives. This film is a slow, orthochromatic film, which allows you to control the contrast both by development AND by exposure. For best results, testing is required. It may not be available in 8x10 sheets any longer, so another film will have to be substituted. Colour sensitivity is not required, so the film can be either ortho- or blue-sensitive.
Another problem you will have is getting enough contrast in your internegative. Your interpositive, if properly made, will be somewhat dark and VERY flat, to maintain all of the detail that was present in the original negative. To produce an internegative which has enough contrast to print, you will need to use Pro Copy and expose/develop for a fairly high contrast. Or, you could try another film which you can develop in a high contrast developer, such as Kodak D-19 or perhaps D-11. Higher contrast film/development will make any dust present all the more noticable, so your interpositive will have to be spotless. A difuse optical system will help in supressing dust.
Good luck; if you have any specific questions feel free to e-mail me, if you wish.