How to get rid of overriding css styles gulp build, postcss processing?

Question

css

1. How to get rid of overriding css styles gulp assembly, postcss processing?
there is css

.b{border: 1px}
...
...
...
.b{border: solid}

This is the first option that I would like to get rid of automatically, and the second option is just a redefinition

.b{border: 1px}
...
...
...
.b{border: 2px}

I would like to get rid of, and what are the ways, maybe plugins for postcss?