How to: A can collide B, C can collide B, but A wont collide C

Lets say I have 10 entities:
1 Hero
9 Boomerangs

The boomerangs attack the hero in a small room. The boomerangs will not collide to each other but can collide with wall and the hero.

How?

Use IGNORE_FLAG2 when moving the boomerangs, and set their FLAG2s.

Thank you so much for the keywords.