Siman 121
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א שׁוֹחִין בְּמוֹדִים תְּחִלָּה וָסוֹף.
1 The Laws of "Modim". Containing 3 S'ifim: We bow in "Modim" ["We are thankful"] at the beginning [of it] and at the end.
ב הָאוֹמֵר מוֹדִים מוֹדִים, מְשַׁתְּקִים אוֹתוֹ.
2 One who says "Modim Modim", we silence [that person].
ג יָחִיד אֵין לוֹ לוֹמַר בִּרְכַּת כֹּהֲנִים. הַגָּה: וְכֵן עִקָּר וְכֵן נִרְאֶה לִי לִנְהֹג, אֲבָל הַמִּנְהָג הַפָּשׁוּט אֵינוֹ כֵן רַק אֲפִלּוּ יָחִיד אוֹמֵר אוֹתוֹ כָּל זְמַן שֶׁרָאוּי לִנְשִׂיאוּת כַּפַּיִם, וְאֵינוֹ נִרְאֶה (בֵּית יוֹסֵף בְּשֵׁם הַמַּנְהִיג).
3 An individual does not say "Birkat Kohanim" ["The Priestly Blessing"]. Rema: 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])