Can I do Singularity to one unit? 12 dmg?

No, you cannot deal 12 damage to one unit with Singularity.

Singularity says "up to two units" — that means you can choose 0, 1, or 2 distinct units. If you choose one unit, it takes 6 damage. You cannot pick the same unit twice to get 12; the targets must be different units if you go for two.