this is not true. Helmet's are designed to absorb a heavy impact, but unless the helmet is specifically made to take multiple impacts, the foam protection will be compromised after one heavy impact.
Dude's right. I have used the same helmet all season, taken quite a few minor head hits, however, 2 weeks ago in mammoth i caught an edge and went straight back and got ROCKED. i got a concussion with the helmet, then i realized the efficacy of the helmet had already been compromised by the other hits. the helmet can be damaged without the damage being visible. i believe that i would not have had the concussion if i had replaced that helmet. (it has no visible signs of impact).
dude, just get another helmet - it's not worth risking getting rocked.