Maybe they failed their morale check when on low health and went berserk? Do they have yellow circles instead of red around their feet? The other possibility is that they're under some kind of spell, like confusion.

None of that happens. When they see each other they start fighting. I thought it might be because they accidentally hit their allies, but AFAIK Baldur's Gate doesn't have the friendly fire mechanic like Doom has.

The dopplegangers in the throne room can go ballistic on each other if/when 'Durlag' throws his aoe spell (Cloudkill?) and they get affected. It turns them hostile to each other and that's the only place I've seen it happen.

Two of them always duke it out in the bedroom. One of them opens with fireball. When they start talking I get out of there as fast as I can.