Yeah, it just means that the eye (the node that is looking) is in the same position as the target. This can happen if the target is the same node, and it can also happen if you are too close to the node you are looking at.
To fix it, just add something like this right before calling look_at
:
if (global_transform.origin.is_equal_approx(look_at_target.global_position) == false):
look_at(look_at_target.global_position, Vector3.UP)
Then it will only look at the node when the positions are not the same.