dnd 5e – Can I cast Aid again to “heal” creatures already under the effect of the Aid spell that have been reduced to 0 HP?

At the time of casting, the second casting is the more potent effect.

The rules for combining magical effects say:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

Now, since the targets of the first aid are at zero hitpoints, the first aid is not currently boosting their current hitpoints by 5, so at the time of the second casting, the second casting is more potent since it would be increasing their current hitpoints when the first instance would not be.

Then after this, the “most recent effect applies” rule would apply and the second casting would effectively become the “active” effect.