76:13 When Shema (which ends with the word “echad,” meaning “one”) is said in the Kedusha of Musaf and people respond, “Echad Hu Elokeinu” (“One, He is our G-d”), this is a mistake because it is forbidden to say the word “echad” (“one”) twice in succession (as this is a practice resembling acknowledging two powers – see Mishnah Brurah 61:22). Rather the congregation should start, “Hu Elokeinu” (“He is our G-d”). The shaliach tzibbur, who waits for the congregation to say their part, can begin with the word echad because there was a break in-between the two occurrences of the word.
76:14 At Mincha on Shabbos, before reading the Torah, we say “Va’ani tefilasi…” “As for me, I pray to You…” – Psalms 69:14). The Sages explain it as follows: the verse that precedes it (69:13) says, “Those who sit by the gate talk about me; I am the subject of the drunks’ songs.” When David followed that with “As for me, I pray to You…” he was saying to G-d that the Jewish people is a unique nation. Even if we were to drink, we would still offer our prayers to Him. We recite that verse before reading the Torah on Shabbos afternoons because even the common people have come out to hear G-d’s words. When yom tov falls on a weekday, we do not read the Torah in the afternoon so this verse is not recited then. However, it is said on Shabbos even if a Torah is not available (or when davening alone – MB 292:2). In such a case, it is recited before the half-Kaddish so that there should not be an interruption in-between Kaddish and the Shemoneh Esrei.