I need to have double line under the header of each page where the chapter title and page number are written (except first pages of each chapter). A sample is as the following picture:
I assume it can be achieved by
fancyhdr package, but don't know how!
Here is a suggestion using package
scrlayer-scrpage. It defines layer page styles. There you can clone the layer that inserts the
headsepline. Then you can shift the layer a bit and change the line width.
fancyhdryou have to redefine