I checked on www.duden.de and it says both versions are correct,
however they recommend schon mal without saying why.
On another site I found something that said schonmal is wrong and that
it should always be schon mal.
Can anyone give a sensible explanation?
Just in comparison: nochmal (short for noch einmal) and erstmal (short for erst einmal) are usually written together (have never seen them separate and I believe this is also considered incorrect). Why then would schon mal / schonmal (short for schon einmal) not follow the same rule?