Some thoughts -
1) Test Neal's thought that the fogging may be coming from something other than the Thomas Duplex - try a fog test with the light off, to make sure that your darkroom is dark when the lights are off.
2) I use an Agfa Sodium Vapour light - which is similar, but smaller than the Thomas Duplex. My lights take a long time to warm up, and until they are warmed up, will fog paper - I turn the safelight on 20 minutes before I start printing to allow it to stabilize - are you doing your test immediately after turning the light on?
3) Are there flourescent lights in the room (even if they are turned off). It may be that the light from the safelight is exciting the phosphors in the flourescent lights and they are emitting light which is fogging your paper.