Duplicated font family names in a font-family property value are either:

Useless and the duplicated font family names can be safely removed
Or it is the result of a bad copy/paste and the duplicated font family names should be replaced with the intended ones

Noncompliant Code Example

.mybox {
  font-family: 'Arial', 'MS Trebuchet', 'Arial', sans-serif;
}

Compliant Solution

.mybox {
  font-family: 'Arial', 'MS Trebuchet', sans-serif;
}