.class.class can also be used avoid the use of !important in case that a higher specificity selector prevents your rule from being applied.

In this case there are not two classes in an element. You just repeat the class which specificity you want to increase, like

(HTML) <div class="something">...</div>

(CSS) .something.something {}

.class.class can also be used to avoid the use of !important in case that a higher specificity selector prevents your rule from being applied.

In this case there are not two classes in the HTML element. You only repeat the class which specificity you want to increase in the style (selector), like

(HTML) <div class="something">...</div>

(CSS) .something.something {}