This is somewhat vague and you may get a better answer if you will clarify as to what sort of implications you are thinking of...
To me, the major implication of this statement is that it claims that Kant is concerned with the impact of our actions on society, not simply with whether they are personally moral. When we use the term "duty" we tend to refer to things that we must do (or not do) because of our place in a society. Duties tend to be our committments to other people -- we have a duty to our spouses and our children.
By saying that not throwing trash is a duty rather than a moral imperative, the statement is implying that Kant would think of this action in terms of its impact on other rather than in terms of our own personal morality.