It's fairly common with autistic children to do this - hearing the same answer / asking the same question again and again can be reassuring / grounding. I suppose it is a sort of stim / echolalia.
Or sometimes a substitute for someone else they can't quite verbalise yet.
I find with my DS he almost doesn't hear my answer unless I give him an answer that doesn't feel right! So then he keeps asking, either way!
First off if it is anxiety related (eg has he just started school / preschool) it may be he's just seeking extra reassurance at the moment and you might just have to go with it to avoid further distress.
Things I've seen suggested in here over the years that might help:
I think it was Polter who once suggested rather than saying 'I don't know' if you don't know, you could try 'I'm 80% sure it's x' - which then if it's not the right answer gives you some wriggle room! I've found this VERY useful!
Especially around things like 'how long will it take' etc
If it is time related use a visual timer so he can check that instead
If it's the same question all the time (my DS asks 'are you happy' all the time - because he can't gauge by my face I think), you could use visual cards with drawings on and direct him to those instead.
Set a time limit on when the next question can be (this doesn't work for us and gets DS more frustrated, but I have seen it suggested elsewhere)
Hopefully someone else will come along with a few more suggestions as well.