While in the middle of a bracha arucha, may I answer amen to that same bracha (said by someone else)?
It is permitted to answer amen in this case.
A number of poskim write that for the same berachah, answering “amen” is not considered a hefsek (interruption), and therefore permitted.
See Shut Binyan Olam no. 5; Shut Yagel Yisrael no. 26-7 (at length) [both base themselves on the Magen Avraham 66:11); Shut Panim Me’iros Vol. 1, no. 59).
Although for a short berachah, for which an interruption is especially stringent, “amen” is not answered even for the same berachah, for a long berachah, such as birkas ha-mazon, it is therefore permitted to answer.