No, this isn't a discussion of the more sweeping Klingon forehead problem. This is simply a problem with Duras' forehead in the episode "Judgement" (ENT).
There is a very strange discrepancy between the flashback sequences where he is on board his ship, and the courtroom scenes. In the flashback sequences his forehead looks normal. The distinctive family ridge that runs around the outside of his forehead is symmetrical.
In the courtroom sequences the ridge seems to be incomplete along the right-hand side of his face (our left). Looking at the screen captures from TrekCore you can see that it isn't likely that his hair is just covering the upper part of the ridge; the hair is actually deeper in his skull than the ridge.
With no evidence of trauma or scarring it's hard to explain away as a battle injury, but I suppose that's a possibility. The real-world explanation would seem to be that sometime between the filming of the flashback and the courtroom scene the prosthetic was damaged and not repaired.
Any real-world or Trek universe explanations/theories are welcome.
Check out the screen captures in context here.