Just some questions regarding the Hatchlings Duplicate ability.

Let us assume a specific hatchling is dead and the corresponding dragon is in play.

1. Can I build another copy of this hatchling into the shadows (This is some sort of general question: Is it possible to build copies of dead uniques into shadows)?

2. If YES to 1: Can i put the hatchling from shadows into play, even with the other copy still in the dead pile, because it immediately moves as a duplicate to the dragon?

Nothing prevents you from putting a dead unique character into shadows; however, you cannot take a unique character out of shadows if there's a copy in the dead pile.

I guess I should also state that the answer to the unasked question is also, "No." If you have the Black Hatchling in your dead pile, you cannot pull another Black Hatchling out of Shadows in order to dupe Drogon.

The reason for this is that the effect that allows you to dupe Drogon will only be active while the Hatchling is actually in play. Because you have a copy in your dead pile, the card in shadows can never attempt to enter play.