Redak: This Elkanah is Ben Achimus ben Amasai ben Elkanah [of the previous verse]. Malbim - below (20), it says that Tzuf's father Elkanah was Ben Machas ben Amasai. This pressed Meforshim (including Radak) to say that Elkanah is Ben Achimus ben Amasai ben Elkanah.

Malbim: Amasai died without children. His brother Machas ¹ did Yibum with the widow, and then he is called Achimus (the brother of the deceased). He fathered a child from her, and called him Elkanah. He is called Ben Machas, for Machas fathered him, and Ben Amasai, for he was called on the name of Amasai to perpetuate his name. This justifies calling Bnei Elkanah Amasai and Achimus, and calling their children Elkanah. It is written "Elkanah Bno" to hint that truly, he is Ben Achimus. It is pronounced Bnei, to hint that Amasai, Achimus and Elkanah are Bnei Elkanah, for Elkanah ben Achimus stood for Elkanah Avi Amasai in place of his deceased son Amasai ² .

Redak: They are Tzuf and Tochi, respectively, mentioned at the start of Sefer Shmuel (Malbim - and also below (19-20), just there the latter is called To'ach).

Redak: Elkanah is Tzofi's father; Tzofi is Nachas' father.