Duplication of selectors might indicate a copy-paste mistake. The rule detects the following kinds of duplications:

Noncompliant Code Example

.foo, .bar, .foo { ... }  /* Noncompliant */

.class1 { ... }
.class1 { ... }  /* Noncompliant */

Compliant Solution

.foo, .bar { ... }

.class1 { ... }
.class2 { ... }