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Stroke width trouble.

I have an art some parts of which have a different stroke width. Even though it appear the same in width field. I`m not sure how to reproduce it, but it is one of basic problems for me and for few artists which I work with. These options
are
off by default, thus I dont think this is about it.
Anyway the main question here is HOW TO SET THE ENTIRE IMAGE FOR ONE SINGLE STROKE WIDTH?
Not only by number but by visual appearance.
P.S. I`m completely sure it is done with stroke width, not with second fill or something else.