What is the function of the 'Frequent' sorting option on the Questions tab?
When many questions are closed as a duplicate of one specific question, that question is a "Frequent" question - that is, it has been asked, in different guises, frequently. In addition, if the question is linked to from another question, it is one of the "Frequently" linked to questions.
In essence the sort is for questions that have been linked to often within the site.
If you hover over the tab, the tooltip is "questions with the most links" - that is, with many questions linking to it.