One should not answer amen after their own brachah, even if they finish their brachah at the same time as the chazzan. However, if the chazan is saying a different brachah, then one should answer amen even if they finish their bracha at the same time. An exception to the above halacha is after bonei yerushalayim in bentching, and for sefardim after yishtabach, shomer yisroel la’ad, and the end of hallel. Therefore if one finishes any of these brachos with the chazzan, one should answer amen.