double-double
Double Trouble
No it's okay! I just think it's always fine to ask. There's a saying from my home-country that's "you've already got a no, but you can always get a yes." Meaning that there's nothing to lose if you ask. It only becomes a problem if someone gets angry, or asks 'why WON't you do it for free?' I've been very lucky to only have received understanding or just silence when I say I cannot!
I think my parents used a quote something like that as well. "Yes means yes, No means no, Maybe means polite no."