Just once you are poisoned or burning you can't get more poisoned or burned.
It's magic! Woohoo!
But seriously, we have to distinguish between the poisoned and burning status effects (which do damage over time) and the direct poison or fire damage.
You can get the status effect only once, true, but you can get damage from the elemental surface multiple times.
If I burn you once with a torch you will get hurt by it, if I burn you again, you will be hurt again.