If a person completed the Beracha of "Hashkibenu" at Arbit, and has yet to begin the Amida, may he answer "Amen" to a Beracha that he hears?
A common case where this question arises is a situation of one who prays Arbit in the synagogue and completes the Beracha of "Hashkibenu" before the Hazzan. He finishes reciting, "Shomer Et Amo Yisrael Mikol Dabar Ra La’ad Amen," and then, a few moments later, he hears the Hazzan complete the Beracha. Should he answer "Amen" to the Hazzan’s Beracha, or would this constitute a Hefsek (inappropriate interruption) between his Beracha and the Amida?
This question also applies to one who, after completing "Hashkibenu," hears the person sitting next to him recite a Beracha, or if he had proceeded far ahead of the Hazzan and he hears the Hazzan recite the Beracha of "Ga’al Yisrael."
This issue is subject to debate among the authorities. The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909), in Parashat Pekudeh, rules that one should not answer "Amen" to a Beracha he hears in between "Hashkibenu" and the Amida. Hacham Ovadia Yosef, however, disagrees, and maintains that one may – and in fact should – answer "Amen" to any Beracha he hears at that point, and this is indeed the Halacha.
Summary: If one hears a Beracha after completing the Beracha of "Hashkibenu" at Arbit, before he began the Amida, he may and should answer "Amen" to the Beracha.