In most dialects of Carrier the subject marker /id/ (Stuart/Trembleur Lake dialect) or /idud/ (Southern dialects), the reflex of the Proto-Athabaskan first person duo-plural subject marker, has become restricted to the dual, evidently due to partial blocking by the extension of /ts'/, the old indefinite subject marker, to first person plural subject. In the Ulkatcho dialect, however, while only /ts'/ can be used in the true plural, /id@d/ retains its original duo-plural usage; its use in the plural is not blocked by /ts'/. Ulkatcho dialect thus presents a counterexample to the strongest formulations of the blocking principle, under which the movement of the indefinite into the first person plural role ought to result in the immediate restriction of the old duo-plural to the dual. Ulkatcho presumably reflects the intermediate stage in the historical development. The indefinite first took on the added role of first person plural, resulting in competition between the two forms.