Why does it say twice that Pelishtim gathered there?
Malbim: Even though recently, Yonason alone overcame the Pelishtim, when the king lost Ru'ach Hashem, Gevuras Yisrael departed, and the Pelishtim were bold to make war with Yisrael. Even before they finished gathering, they went to Yehudah, and finished gathering there, as if they already conquered it.
What is "Efes Damim"?
Rashi: It is the name of a place. Radak - in Divrei ha'Yamim 1 11:13 it is called Pas Damim.
Radak citing R. Yochanan (Midrash Shmuel 20:1): It was a red field (like blood). 1
Radak citing R. Shmuel bar Nachmani (Rus Rabah 5:1): From there, Pasku (Yerushalmi Sanhedrin 2:5 - Nifseku) Damim. 2