Why did he say "Gam" twice?
Rashi (from Midrash Shmuel 20:5): These include - there were two cubs with lion, and two cubs with the bear.
Radak: Each "Gam" includes the other. This is like "Gam Tzonchem Gam Bekarchem" (Shemos 12:32).
Radak (citing 32 Midos of R. Eliezer, 3): These are three inclusions - there were two cubs with lion, and one cub with the bear.
Malbim: I killed two animals - all the more so I can kill the Pelishti, who is no better than one of them!
It says "Ki" - does the end of the verse explain the beginning?
Malbim: Yes. If you will say that a man is more dangerous than a beast due to his intellect and form, after he reviled Elokim's legions, he descended from human to animal.