Paam Achas B'yom Bchol Yom

We've mentioned in the two previous posts an excellent shiur by Rav Hershel Schachter on the issue of earning a living according to halacha (found here).

In the course of the shiur Rav Schachter mentioned a general rule that if a mitzvah is said in the torah and no time is given for the mitzvah then the assumption is that the chiyuv for the mitzvah is once a day every day. [I'm not sure if Rav Schachter means that this is an actual "rule" or just more like a "rule of thumb" but in any case I will briefly analyze what he said.]
Rav Schachter cited two proofs (or maybe examples) of this rule. One is from the Rambam's shitah that the mitzvah d'orayssa is to daven once a day every day. We see from this that a mitzvah (i.e. davening) which the torah doesn't give a time for, the assumption is once a day every day.

The second proof came from Nesius Kapayim. In O.C. 228:3 the halacha is brought as follows (rough translation):

If [the cohen] already ascended once on this day, he no longer transgresses an aseh [if he doesn't ascend the duchan] even if they call him up [to duchen].
Once again we see from this halacha that the assumption is that the mitzvah of nesius kapayim is once a day every day.

After doing a bit of research, this proof seems somewhat difficult to me. This halacha comes from a gemara in Rosh Hashana 28b and a tosafos there. The Chasam Sofer in O.C. teshuva 22 points out that if a kohen was already called up for shacharis and he were then called up for mincha he would certainly be chayav to duchen. (Generally, we don't duchen at mincha because of shichrus.) Also, it is pashut that such a kohen would make a blessing on the duchening, even though he already made a bracha at shacharis. The only issue is if a different minyan of shacharis mispallelim call up the kohen. Then we can ask if he is chayav to duchen and whether he should make another bracha.

This pshat of the chasam sofer would seem to go against the assumption that the chiyuv of nesius kapayim is once a day every day. On the contrary, if we called up a kohen for mincha he would be chayav to duchen again!

Further, we can now raise a question. We know that on Yom Kippur we duchen more than once. We also make a bracha on every duchening. If the actual chiyuv to duchen is only once a day every day, why make a bracha on each duchening??