1)
It already said "v'Rak Hi Yechidah." What does "Ein Lo Mimenu" add?
1.
Radak citing Targum Yonasan: It is as if it said Mimenah; he had no grandchildren from her, for she did not marry yet - "Lo Yad'ah Ish" (verse 39). Also Mesores says so (it is as if it says Mimenah).
2.
Radak: Perhaps Yiftach's wife had other children from before she married him, and Yiftach raised them and they were considered like his children, but from him, she was his only child.
3.
Malbim: Sometimes one is called Yachid even if he has others, but they are less important, e.g. "Es Yechidcha? Yitzchak" (Bereishis 22:2). "Ein Lo Mimenu" teaches that he had no other children at all.
