Thanks Jez for you answer. I'm very sure it isn't debounce.
When I don't read the I/O expander when I get an interrupt the int routine is called once.
The signal on the Int pin on the expander is a copy from the switchsignal.
When I do read the expander when I press a switch the signal on the int pin changes. It isn't a copy anymore of the switch state
I've downloaded the Max6818 datasheet. Thanks fo the tip but I prefer to solve this in software. I have 28 switches to read. Sorry I didn't mention that