When do we use koho vs kdo?
The preposition "na" requires the accusative case, therefore we use "koho". The "kdo" pronoun has different forms for different cases.
Mohlo by být pouze "koho nebo co čekáš"?
Ne, to je expect.
V hovorové češtině je to běžně používáno.
It's normally used in spoken czech.
I didn't claim it is not correct Czech. But it means something else.
In English, using the preposition 2 x would sound odd. ("For whom or for what are you waiting for" vs. "For whom or what are you waiting for"). In Czech, is it considered more proper to use "na" twice or would one "na" suffice for both? words?
I find it better to repeat it but it can be left out.