@Leen1 --
Since you must file as a full-year resident of both NY and DC, ALL your income is taxable by EACH state, regardless of its source. Thus "sourcing" is irrelevant in a dual resident situation.
Sourcing refers to the common situation where a taxpayer is a resident of one state but has income from another. For example, a NY resident who works in NJ (but is not a resident of NJ) would have NJ-sourced income.
You can claim an "other state credit" only on your NY return, since NY is your domiciliary state. This is what the DC tax instructions are telling you.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
