Double rasterization work on triangles edges. I know that fragments are rasterized in a 2x2 quad. That's why the seam between two triangles can be rasterized twice. At least that article states this. I wonder is it still true today on modern hardware?
That's why the seam between two triangles can be rasterized twice.


The seam won't be rasterized twice because of the OpenGL or D3D fill/rasterisation rules. 

What I think Humus is alluding to is that a fragment shader may be executed multiple times on pixels spanning the shared edge, which just implies wasted effort.

The moral of the story is "avoid small or long/thin triangles if you want efficient rendering".