Ross, Motive and the Sense of Duty

In a revisitation of Ross' The Right and the Good, I have been strangely trying to figure out a decent representation of the following textual argument. Here is that little snippet that catches my fancy:

Those who hold that our duty is to act from a certain motive (Kant is the great exemplar) usually hold that the motive from which we ought to act is the sense of duty. Now if the sense of duty is to be my motive for doing a certain act, it must be the sense that it is my duty to do that act. If, therefore, we say 'it is my duty to do act A from the sense that it is my duty to do act A.' And here the whole expression is in contradiction with a part of itself. The whole sentence says ' it is my duty to-do-act-A-from-the-sense-that-it-is-my-duty-to-do-act-A.' But the latter part of the sentence implies that what I think is that it is my duty to-do-act-A simply. And if, as the theory in questions requires, we try to amend the latter part of the expression to bring it into accord with the whole expression, we get the result 'it is my duty to do act A from the sense that it is my duty to do act A from the sense that it is my duty to do act A,' where again the last part of the expression is in conflict with the sentence as a whole. It is clear that a further similar amendment, and a further, and in the end an infinite series of amendments would be necessary in the attempt to bring the last part of the expression into accordance with the theory, and that even then we should not have succeeded in doing so. (R&G, p. 5, CH 1).

This seems, at first glance, to be a devastating argument. Motive and duty cannot be contained in the other. However, I am unsure as to how the contradiction is reached given that the contradiction is internal to the statement. And how exactly does Ross understand the given expression? Untangling this mess will require some effort, and has been the subject of some scholarship since this is a famous argument made against Kantians (Arthur T. Shillinglaw 1933 in Mind for starters). Before we get to analysis of this argument, I want to further add what he says about acting from the sense of duty.

On this argument, Ross clearly says that only acting from sense of duty will it lead to a infinite regress (p. 6). However, acting from other motives will be free from the infinite regress. Still, he reminds "it would be paradoxical to hold that we ought to act from some other motive, but never ought to act from a sense of duty, which is the highest motive." For Ross, there is a positive significance acting from a sense of duty has; it is just that we cannot regard any theory which holds "that motive of any kind is included in the content of duty" (p. 6)

Now, reductios attempt to get A and Non-A statements together in a proof. In application, this means showing that a theory or thesis commits one to its negation as well as positive formation. Above, let's identify what thesis is up for attack. I'll call that the Kantian Containment Thesis, or KCT for short.

KCT: The motive from which a moral agent ought to act is the moral agent's sense of duty.

So, Ross would show us how we reach ~KCT:

~KCT: The motive from which a moral agent ought to act is not the moral agent's sense of duty.

Yet, I am not too sure that this is done. Any thoughts? I could be missing something, but I'm going to think more on this.