With Sharpie markers, (and other similar markers) you can get a very smooth surface.
The point is not that duplication should be avoided. On the contrary, the more layers you make during the coloring, the more even the surface will get. Apply the new layer always as a part of the previous one, and even on top of it. Continue that way, filling the entire surface, and there will be no unpleasant blotches.
You can also color the entire area once and then the whole area again. It does not matter if the color dries in the meanwhile.
In any case, color the paper as many times as it takes to saturate the paper. Check from the other side of the paper if it is saturated enough.
Upper area is colored with just a single layer. The lower area is colored so that the paper is saturated, which took about three layers of color.
A saturated coloring requires a little more effort, but the coloring becomes much more meaningful when the outcome is wonderfully smooth and the color has been properly absorbed into the paper.
In the finished picture clouds and the sky have been colored using a Sharpie the way I have described here.
Book: Mental Images vol 2