What exactly is a 'Duplex appartement'?

I know it's an appartment of some type but what do they mean with 'Duplex'?

In the Netherlands a duplex is half a house.
Many houses where build in time of housing shortage, and temporary they where split in two different apartments.
The idea was to make them into whole houses after a while by combining the two apartments.
After 40 or 50 years the houses are nearly always still split.
So if your question is in the wrong category, it will be a small apartment in an older house.

The dictionary gives for Belgium: An apartment over 2 stories of a building with a staircase inside.

It has an upstairs bit or a bed deck. It's not all on one level, but it's still a flat/apartment

duplex is where you are attached to another appt right next door to you..

It is an apartment in a house where the separate apartments are like mirror images of one another.

One which is on two floors.

A duplex apartment is simply an apartment that is 2 stories high. Normally the living area, kitchen, maids room is located on the floor your front door is on and the bedrooms are upstairs.

if you don't want a duplex they have triplexes too !

In the UK, we would call them a maisonette. It's basically a house in a block of flats. The duplex apartment has a lower floor with kitchen, living room etc but the upper floor has the bedrooms (rather like a house). The duplex apartments would be stacked up one above the other like flats/apartments.

it's an appartment that's located on 2 floors. Usually you'll find kitchen, living room and one sleeping room on the main floor, and upstairs a second sleeping room + bathroom.

A duplex is in fact a apartment with a indoor stairs that lead to a other floor sometimes with a roof terras.