"Duty" and "obligation" are "distinct concepts"? Really? Let's see what Dictionary.com says:
du⋅tyOr perhaps the OED:
1. something that one is expected or required to do by moral or legal obligation.
dutyClearly a hugely important "conceptual" distinction!
1. a moral or legal obligation.
I really hate having to do this. Because the difference is so utterly clear and obvious to people who don't have an amazingly blinding bias against any subject whatsoever, he seems to turn off that aspect of common sense, in which no body seems to notice because the effort to put into words something so ridiculously elementary. I will now attempt to explain the difference between "duty" and "obligation" to Mr. Nyquist.
Firstly, it makes absolutely no sense to compare the same words on two different dictionaries, finding one to be necessitate circular reasoning of the other. If one wished to ascertain the difference between two words, you would use the exact same source. From dictionary.com:
Obligation: 1. something by which a person is bound or obliged to do certain things, and which arises out of a sense of duty or results from custom, law, etc.
Assuming Mr. Nyquist can read things that deviate from his senseless bias, (which is certainly against all the evidence so far), there is a distinct differential aspect of the words "duty" and "obligation". While denotatively, the words lack sufficient distinctions, connotatively, the differences are quite clear. As duty only suggests something that is required to be done by an entity, (dropping contexts in the process), obligation presupposes a specific cause or reason for this requirement. There is no intrinsic duty by one to help others in desperate need, but there is an obligation, leading to their specific nature in relation to their ethical value to the person who would be saving the one in need. It arises, (as stated by Dr. Peikoff), from the fact that other people are needed for one's survival, as well as the fact that one could never have a voluntary moral claim to the help of another person if here were not do offer the same. While the concept of duty needs no rational and objectifiable source, (a la Kant), obligation does.