Hi Kevin,
I suggest checking your flat image in this area. This double line effect looks typical of that produced by a dust donut. Take a profile across the flat at the location of the spectrum. I was discussing with Jack Martin the other day who saw something similar (at a different location). It turned out to be a defect visible in the flat which for some reason we don’t quite understand was not corrected by the flat (The defect was weaker in the flat than in the image)
Cheers
Robin