Don't know about duty but I think everyone has a right to expect help and a right to give help. It makes for a better world where these rights are regularly exercised.
This might make sense when asking a stranger for directions or for someone to help with your car problem, because the relationship is 1:1.
But this breaks down in contexts where scale is a factor.
When assisting users of your project, the relationship is 1:many. And if the project is popular, it is unfair to expect the author to be helping so many people.
Of the people with whom you have formed strong social bonds, yes.
Of random strangers? .. I don't think so.
Unless they are bound by some sort of a contract (employees, servants, etc).
The alternative brought to it's logical conclusion is Shkreli and rasing medicine prices by thousand percent.