Shulchan Arukh, Orach Chayim 121/שולחן ערוך·אורח חיים קכ״א

1The Laws of "Modim". Containing 3 S'ifim:We bow in "Modim" ["We are thankful"] at the beginning [of it] and at the end.

2One who says "Modim Modim", we silence [that person].

3An individual does not say "Birkat Kohanim" ["The Priestly Blessing"]. Gloss: And this is the principle, and it appears to me that [people should] practice like this. But the the widespread custom is not like this, rather even an individual says it any time it is appropriate to "spread the hands" [i.e. to say Birkat Kohanim], but this does not appear [correct to me]. (Beit Yosef in the name of Manhig [Sefer haManhig by Rabbi Avraham ben Natan, 13th century Provence])