Interestingly, I also managed to perform the same trick with the Duplex by using an inexpensive sheet of Roscoe Roscolux #19 (Fire) filter material as a cutoff filter.* The emission spectra for a low-pressure sodium vapor lamp includes numerous additional spikes in the blue and green portions of the spectrum. You don't see them with your eyes, but the photo paper sees them. Get rid of them and you can significantly increase the safety of the safelight.
* See the technical data sheet for this filter here. The transmission graph demonstrates why this material is so effective as a cutoff filter for the higher frequency blue and green spikes. I looked at all of the possibilities and this color was the best choice.