Double rasterization work on triangles edges.
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".