In a Nusach Ashkenaz minyan/shul, there are certain individuals who (not infrequently) arrive late for Ma'ariv, and therefore miss Borchu at the beginning. Certain of these people have some kind of background in Nusach Sefarad. Therefore at the end of Ma'ariv they say Borchu again on their own initiative.
The main question is: Is this appropriate from the perspective of the halacha/minhag of Nusach Ashkenaz?
As a secondary question: Also, when those same individuals arrive late for Shacharit and miss Borchu there (including on Sunday, Tuesday, Wednesday and Friday), they never repeat Borchu then. As far as I know Nusach Sefarad does not have a second Borchu in Shacharit, so what is the justification for saying a second Borchu aloud at Ma'ariv and not Shacharit?