Why is this an issue?

Having the import of the same file twice, makes one of them useless. Leaving them in reduces the code’s readability, since their presence can be confusing.

Noncompliant code example

@import 'a.css';
@import 'a.css'; // Noncompliant

@import url("a.css");
@import url("a.css"); // Noncompliant

Exceptions

This rule ignores @import in less files.