Hedron lanterns fend off more than just the darkness.

Lantern Scout

Rally — Whenever this creature or another Ally you control enters, creatures you control gain lifelink until end of turn.

Battle for Zendikar

Rally — Whenever Lantern Scout or another Ally enters the battlefield under your control, creatures you control gain lifelink until end of turn.

Battle for Zendikar Promos

If a creature with a rally ability enters the battlefield under your control at the same time as other Allies, that ability will trigger once for each of those creatures and once for the creature with the ability itself.

Multiple instances of lifelink on the same creature are redundant. There usually isn't much benefit in having the rally ability resolve more than once in a turn, other than that new creatures will be affected by the ability resolving an additional time.

When an Ally enters the battlefield under your control, each rally ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve.