What you're really trying to do with a low-pressure sodium (LPS) safelight like the Duplex is to remove the residual blue and green light that is emitted as a consequence of the argon and neon intentionally introduced into the tube during manufacturing. These gases assist in the striking of the lamp, and help account for the weird colors you see at initial startup before the sodium color takes over.

It's these not-visible-to-the-eye blue and green spikes that are the source of the Duplex's reputation for fogging paper, not its overall brightness. See here for an example of the output spectra from an LPS tube. (Fourth example down the page.) Note the much dimmer blue and green bands to the left of the predominant yellow. There is also a red band to the right, but that of course does no harm.

The Roscoe #19 Fire gel will transmit about 35% of the sodium yellow, while blocking essentially 100% of the blues and greens. And that 65% reduction in intensity of the yellow is usually welcomed by most people with small darkrooms. Often it's still hard to see the image on the easel even at 35%, so the vanes must still be closed while focusing. This is what I must do. See here for a transmission graph of the Roscoe #19 filter gel.