Question about recasting buffsFollow

When new people join your party I'm assuming that you need to recast the buffs in order for the new people to get them. When I try this, I get a "Not enough concentration" message. So I end up having to cancel all of my existing buffs and recasting them all. Is this what you're supposed to do?

It seems like recasting buffs should automatically cancel out the existing identical buffs and replace them. Why does casting the same buff require more concentration? Does recasting the same buff actually strengthen the existing buff, and that's why more concentration is needed?

Pardon my noobness.