No back-to-back heroism is a big nerf to 5v5 shaman teams. Any priest with a tiny amount of skill will mass-dispel heroisms, which is why it needs to be recast several times. I would hope that blizzard implements this so that:

1. Another heroism can not overwrite a currently active heroism buff.
2. Only when heroism effect expires after its full duration do you gain the debuff that prevents the effect to be repeated.