1)
It already said above (15:20) that he judged Yisrael for 20 years!
1.
Radak citing Yerushalmi Sotah 1:8: The Pelishtim feared him 20 years after his death, just like they feared him 20 years in his lifetime.
2.
Radak: That was said after the salvation in Ein Korei. It is as if he died then, for he did no salvations after that; his demise began. Now that he truly died, it says that he judged Yisrael, like for every Shofet.
3.
Metzudas David: It was repeated to teach that the time he was incarcerated count towards the 20 years that he judged.
