These are duties, such as the duty not to commit suicide and the duty not to make a lying promise, which admit of no exceptions. (It is not okay to make lying promises as long as you don't do it much.)
These are duties, such as the duty to help people in need, which do admit of exceptions. They are still duties, but you have some choice about how to fulfill them. (You don’t have to give all your money to every charity; your inclinations can enter into which charities you choose to give to.)