If, in survival mode, the player throws a boomerang, they can still use the one in their hotbar to attack mobs or break blocks while the boomerang entity is out. One would expect that the boomerang wouldn't be able to be used as two weapons at the same time in this manner, and wouldn't be able to use it as a melee weapon while the projectile entity is out.

This game breaking problem is still present.
You can walk up close to an enemy and spam boomerang, causing damage without cooldown. Which makes this weapon too overpowered.
Solution? Implement weapon cooldown, as with swords, axes etc.