Joshua 8:2 is God's command to set an ambush behind Ai; verse 12 executes that with 5,000 men on the west.

In Joshua 8:1, God commands taking all people and promises victory; verse 12 shows part of that obedience—setting an ambush.

Joshua 8:13 summarizes battle positions, including the liers in wait on the west, which verse 12 detailed as the 5,000-man ambush.

Joshua 8:3 describes 30,000 men for ambush; verse 12 adds a separate 5,000-man ambush, complementing the strategy.

Joshua 8:9 records the 30,000 going to ambush; verse 12 introduces another 5,000 ambush on the west, a different group.