I have said demandi demandon before and no one has ever corrected me. However, it always sounds awkward to me to have these two words together and I always wonder if people think I sound like an idiot saying it this way.
Well, I just read starigi demandon. Is this a better way to say it? If not, what is the correct way?
Is "demandi demandon" even correct at all?
Demandi demandon to me means "to ask a question something." One way to say "to ask a question" is simply demandi. What else are you going to ask other than a question?
The verb I have always used with "demandon" as the object is starigi. I was not familiar with fari demandon (although in 20 years, I must have heard it multiple times.) My impression from looking through modern texts in tekstaro.com (e.g. artikoloj de monato)is that "starigi demandon" is at least slightly more common if not clearly so. (11 variations of fari demandon and 13 variations of starigi demandon using the same search methods.)
I'm sure all three are fine.
Usually, the object of the verb demandi is the person being asked, e.g. Li demandis min pri la historio de la loko.
In Tekstaro, if you search "demandon", you will find several examples of fari demandon meaning "to ask a question". E.g. permesu, ke mi faru demandon. This is slightly different than starigi demandon which means "to pose a question".
Demandi demandon is grammatically correct, but I understand the desire to avoid redundancy of the same root being repeated. One option is to rephrase it slightly. Instead you can "ask something": demandi ion.
According to the PIV, one can fari demandon, meti demandon, direkti demandon (al iu); all these usages are from the works of Zamenhof. Fari demandon seems the closest to generic English 'ask a question'; the other two I would translate as 'put a question' and 'direct a question (to someone)'. I would say that demandi demandon is probably to be avoided.
BTW, the PIV (Plena Ilustrita Vortaro) is accessible online at http://vortaro.net. It's a wonderfully useful resource and now thanks to the interwebs quite easy to consult without hauling out a heavy and expensive book.
Ĉu mi povas demandi pri io? in some cases will do too. Other languages will often use such a form.
Ĉu mi povas fari demandon? in other languages use a help verb stellen (de, nl), poser (fr).
The English form with its semantic redundancy is an exception.
