It is customary in many communities to recite personal tefillot after the silent amida. Canonized in many nusach Ashkenaz/Sefard siddurim, this is followed by the final line of kaddish (based on the verse in Job) "עושה שלום במרומיו הוא יעשה שלום עלינו ועל כל ישראל. ואמרו אמן" - "He who makes peace in His heights, should make peace upon us and upon all Israel. And say 'Amen'." The latter clause "and say Amen" seems to be an exhortation to the congregation which seems inappropriate to a personal, silent prayer.
Do any commentators discuss whether or why this was included in the standard Ashkenaz/Sefard siddurim? Is it a typo carried over from the kaddish? Are there extant prayer books that omit the phrase?