no-duplicate-selectors

Disallow duplicate selectors within a stylesheet.

    .foo {} .bar {} .foo {}
/** ↑              ↑
 * These duplicates */

This rule checks for two types of duplication:

Duplication of a single selector with a rule's selector list, e.g. a, b, a {}.
Duplication of a selector list within a stylesheet, e.g. a, b {} a, b {}. Duplicates are found even if the selectors come in different orders or have different spacing, e.g. a d, b > c {} b>c, a d {}.

The same selector is allowed to repeat in the following circumstances:

It is used in different selector lists, e.g. a {} a, b {}.
The duplicates are determined to originate in different stylesheets, e.g. you have concatenated or compiled files in a way that produces sourcemaps for PostCSS to read, e.g. postcss-import).
The duplicates are in rules with different parent nodes, e.g. inside and outside of a media query.

This rule resolves nested selectors. So a b {} a { & b {} } counts as a warning, because the resolved selectors end up with a duplicate.

Options

The following patterns are considered warnings:

.foo,
.bar,
.foo {}

.foo {}
.bar {}
.foo {}

.foo .bar {}
.bar {}
.foo .bar {}

@media (min-width: 10px) {
  .foo {}
  .foo {}
}

.foo, .bar {}
.bar, .foo {}

a .foo, b + .bar {}
b+.bar,
a
  .foo {}

a b {}
a {
  & b {}
}

The following patterns are not considered warnings:

.foo {}
@media (min-width: 10px) {
  .foo {}
}

.foo {
  .foo {}
}

.foo {}
.bar {}
.foo .bar {}
.bar .foo {}

a b {}
a {
  & b,
  & c {}
}