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
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?