Can you slab over slabs?

Dry_Rot

Just make sure that you don't bring the finished ground level nearer than 150mm below the damp-proof course (if there is one). Doing so could cause damp problems in the external walls in the future.

Bryan

Mankysteve

If its up against the house make sure it doesn't block any of the air ducts or make water flow into them.

iamcornholio

The two main issues with paving over existing paving are

making sure that there are no voids between the two layers which may become a home for things like rats or mice etc

making sure that any water with permeates the top layer is not held by the bottom layer or made to drain/collect in isolated places

Also, just having a sand base may mean that the sand stays too wet between the two layers and so the top paving sinks in main traffic/use areas. Drain holes would help to let water through the bottom paving