Saying "Amen"

Does one say amen to yishtabach if they finished the Bracha at the same time as the chazzan?

Shalom!

Thank you for your question.

As a general rule, one never responds “Amen” to a blessing which one has personally recited. Indeed, doing so is deemed by our sages to be reprehensible.

There are, however, a number of exceptions. For Ashkenazim, the only exception to this rule is following the blessing “Bonay B’rachamav Yerushalayim” in the Birkat Hamazon in which one is to respond “Amen” to one’s blessing. The reason for this is in order to differentiate between the sections of the Birkat Hamzaon until this point which were Biblical in nature and the remaining sections which are Rabbinical in nature.

Sefardim have the custom to respond “Amen” to their own blessing in a number of additional places as well, such as at the conclusion of Hallel, Yishtabach, and Shomer Amo Yisrael La’ad at Maariv. This is because responding “Amen” at these points reflects the idea that within Hallel and Pesukei D’zimra all interruptions are forbidden with the exception of liturgical responses. Similarly, these concluding blessings each represent the completion of a distinct section within the prayers.

In the event that one has concluded the blessings of Yishtabach, Hallel, and Shomer Amo Yisrael La’ad simultaneously with the Chazzan, one is to respond “Amen” to the Chazzan’s recitation even though it may appear as if one is responding “Amen” to one’s own blessing even according to minhag Ashkenaz. This is because these three blessings each represent the conclusion of an entire section which, as mentioned, according to some authorities, warrants one to respond “Amen” to one’s own blessing anyways.

SOURCES Brachot 45b; OC 215 with commentaries