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 violation, because the
resolved selectors end up with a duplicate.
Options
true
The following patterns are considered violations:
.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 violations:
.foo {}
@media (min-width: 10px) {
  .foo {}
}.foo {
  .foo {}
}.foo {}
.bar {}
.foo .bar {}
.bar .foo {}a b {}
a {
  & b,
  & c {}
}Optional secondary options
disallowInList: true | false
(default: false)
This option will also disallow duplicate selectors within selector lists.
For example, with true.
The following patterns are considered violations:
input, textarea {
  border: 2px;
}
textarea {
  border: 1px;
}