"If someone gave me a penny each time I unlocked Default backpack, i would have two pennys. Which isn't a lot, but it's weird that it happened twice"

Basically, you can /unlockbackpack any of those two default backpacks, but only once. Weird.

Basically the two default backpacks are unlocked but not registered in the player data that they are unlocked. Not going to worry about about preventing this.