The change that passed in front of Freyja's eyes changed everything and she found herself stepping backwards in genuine shock, the mirror image of herself now standing there. When the thing spoke, it was with Freyja's own voice, and when it (she?) hit Freyja, she was pushed further back by the power behind it and the shock.
But there was no time to think, no time to consider if this was some sort of vardøgr that had infiltrated them all. Her focus now had to be entirely on fighting.
On one hand it was easier: there was no doubt that this thing was not her, and so Freyja had no lingering part of her worried about hurting it.
On one hand it was harder: the thing seemed to fight in the same ways that she did, seemed to anticipate her moves.
But that second hand also meant that Freyja could anticipate what it might do, and the fighting - as brutal as it had to be - required quick thinking strategy, required Freyja pushing against her first impulses - that punch leaves the solar plexus exposed and hitting it would be the smartest move - and picking a different route -kick the back of the knee out. It's less debilitating but less expected.
One of the vardøgr's strikes to her face had cut her cheek open, and Freyja could taste the blood. Another punch aimed for her and Freyja spat "skjǫldr!", calling up a shimmering shield of golden light to block it.
She suspected magic would serve her little advantage - if the creature knew Freyja's mind and her body, it surely knew her magic as well. She was grateful to not know any magic that could instantly kill a person.