If you rez a second copy of a unique card, the first rezzed copy is trashed. Cards in either player's score area are not installed and so are not affected.
Actually, it's being active that matters, not necessarily just being installed.
Page 14, Column 1, Paragraph 6, Core Rules wrote:
While an agenda is in the corporation's score area, it is active and adds its agenda points to his score.
Page 18, Column 2, Paragraph 1, Core Rules wrote:
While an agenda is in the runner's score area, it adds its agenda points to his score.
Page 11, Sidebar 2 wrote:
There can be only one unique card of the same title active at a time. If a card with a unique title becomes active, any other card that shares its title is immediately trashed. This trashing cannot be prevented.
So if the corp had some kind of way to score executives, then rezzing another one would trash it from the score area. Which is pretty weird. And probably why the corp can't score them.