Most people think/know that Irenaeus's reference to "the Secundians" (Σεκουνδιανῶν) is a mistake. But Epiphanius thinks they are a real sect and one known to two different Church Fathers:
Next the authors who had written the truth < about > these people so well refuted < them > in their own treatises–Clement, whom some call Clement of Alexandria, and others, Clement of Athens—(2) and St. Irenaeus besides (who), to poke fun at that tragic drama of theirs, quoted those words and raised < the cry of > (3) “Alas and alack!”
Clement is mentioned because the Alexandrian identifies Epiphanes as a Carpocratian
“I have quoted these remarks,” the author who wrote against them < says >, “in refutation of those who do not live rightly,” and the Basilideans and Carpocratians, and those who are named for Valentinus and Epiphanes, with whom the Secundus whom I placed before him, was associated.
Not sure how this makes Clement a witness to Secundus or the Secundians.