It says it has multiple attacks rolls, but does not do damage on every hit. Twin Strike for the Ranger states that it does damage on every hit.
Brutal Barrage has this
Attack: Constitution vs. AC. Make the attack three times.
Hit: Constitution modifier damage.
Effect: If you hit the target two or more times, it falls prone.
So it seems that the extra rolls are only there to improve your chance of hitting and only creates an effect if it hits more than once, but it always does Con mod damage only once.
Twin Strike says this
Attack: Strength vs. AC (melee; main weapon and off-hand weapon) or Dexterity vs. AC (ranged), two attacks
Hit: 1[W] damage per attack.
I think the per attack is the difference maker and since Brutal Barrage does not say Con mod damage per attack, it only does the Con mod damage once.
I believe I'm reading this correctly, but I noticed the Char Op seems to think differently as most of the builds depend on damage per hit. I think I could find more examples of this as well...I was just wondering if this has ever been noticed.