Preventing “double” borders in CSS

Question

Say I have two divs next to each other (take https://chrome.google.com/webstore/category/home as reference) with a border.

Is there a way (preferably a CSS trick) to

Answer 1

Using Flexbox it was necessary to add a second child container to properly get the outlines to overlap one another...

SCSS

.grid__container {
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;
    margin: 0 1px 0 0; // margin-right 1px to give the correct width to the container
}

.grid__item {
    flex: 0 1 25%; // grid of 4
    margin: 0 0 1px; // margin-bottom to prevent double lines
}

.grid__item-outline {
    margin: 0 0 0 1px; // margin-left to prevent double lines
    outline: 1px solid #dedede;
}