“one time.” Meaning, in one military campaign, not on the same day or week. This was a huge campaign that started in Joshua 10. Furthermore, some of these cities were recaptured by the Canaanites. From Joshua 10:43 we learn that the army of Israel returned to Gilgal, and that opened the door for the Canaanites to return to their cities. The army of Israel did not disband and thus begin to settle in the cities they had conquered until the whole land was conquered. Note that in Joshua 10:43 the army returns with Joshua to Gilgal after the battle; it does not split up and occupy the cities that it just conquered.
“Yahweh the God of Israel fought for Israel.” Yahweh’s fighting for Israel and thus giving them the land is one of the themes in Joshua.