If he holds dual citizenship- he can’t be president.
however- it may (or may not) depend on when and under what circumstances he revoked his dual status .
I think what it would do is harden the lines between the marxist wing of the DNC and...the rest of America. I can’t begin to guess at the legalities involved- but it would mean the end of his candidacy, practically if not literally.
That is...if he revoked it. And just who will investigate this I wonder. ;)