Help identifying type of plasterboards

[Loguc]

Hi all.
I'm planning to put up some shelves on walls with plasterboards.
I've already done that on another wall, a perimetral one made up as 12mm boards - cavity - bricks.
I drilled a couple of exploration holes into an internal wall (in spots where the wall was sounding hollow), and it seems two different types of plasterboards have been stacked with the resulting thickness being just short of 30mm. I'm attaching a couple pictures I managed to take.

The material making up the board on the surface seems like the one in the rest of the house, that is to say grey and sandy-ish, while the inner board seems whiter and chalky-ish.
The house was completed in 1990 and I believe all the walls are in the original state.
Can anyone identify what sort of work has been done here? Can these really be two different types of plasterboards?
Thanks.

 

[^woody^]

Why does it matter?

 

[dontbelieveawordofit]

pass. the photos aren’t good enough.
the house wasn’t completed until 1990 so technically those sheets could contain asbestos , although highly unlikely.
it’s more likely an insulation board of some sort on external walls.
you can get a test kit on ebay if your’e bothered.
stop making dust out of it and drill what you have to to get your shelves up.

 

[Loguc]

The double plasterboards are in an internal wall, not an external one.
On the other side there's a bathroom (with no windows). May this be the reason why there's an extra layer?