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 problem, because the resolved selectors end up with a duplicate.

Options#

The following patterns are considered problems:

.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 problems:

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

.foo {  .foo {}}

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

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

This option will also disallow duplicate selectors within selector lists.

For example, with true.

The following patterns are considered problems:

input, textarea {  border: 2px;}
textarea {  border: 1px;}