On Rosh Hodesh, before hallel, the shaliach says the bracha on hallel and the kahal answers amen and then repeats the bracha. Why is this bracha treated differently than most others. Typically, if someone else says a bracha and you say amen you are covered by their bracha.* (Yes, the person leading the davening can have the intent not to cover you, and you can have the intent not to be covered, but that is an unusual approach.) Why do we take this unusual approach to the bracha before Hallel? (All the most so because Hallel is only rabbinic in nature.)
*For example, on Rosh Hashanna the person blowing shofar says the bracha :hear shofar" and we say amen and do not repeat the bracha.