[Taqwus]

Protection does not stack; once Mass Protection has been cast, Barkskin / Protection are pointless unless additional units have been gained by summons, charm / enslavement etc.
I'd be happy if the spell AI would refrain from committing pointless fraticides, such as casting Breath of Winter when surrounded by hordes of non-cold-immune friendlies, or casting Protection on friendlies surrounded by heat-aura folks.