If a unit already has the Invulnerability ability and still is within range the Invulnerability ability will be added again.
A problem that might occur. If a unit already has the Invulnerability ability and still is within range the Invulnerability ability will be added again. I'm not sure if the unit then will have 2 of those abilities or if it will simply ignore that unit when adding.
Abilities will not be added twice.
One could use an aura that only gives a buff. Pick units in range that have the buff. All other filtering is done in object editor in Targets allowed. This way you don't have to use a long matching this and matching that and mathching whatever line.
Use a custom invulnerability ability so the aura does not clash with other possible invulnerability abilities.