Sorry, I don’t think I understand what you are doing exactly. Extrusion doesn’t appear to create double vertices when I try it.
But I have not really used GP yet, so that’s probably just me; I don’t know what Additive Drawing is (I know it is a thing, just don’t know what it is…I switched the Blend to Add…).
How did you edit a line to have two points? (Dissolve the inner points, or draw a line with 0 Subdivisions, or….)
You do know that GP already has a Build Modifier, right?
