[lch]

As far as I am aware, the Visitor is no hero or multihero. He is a unique being that comes from the Void Gate. What you wrote is exactly the reaction that the game gives when a unique unit is being moved, through a Wish for example. It's strange that the game didn't check if he already was under your control, true. But I don't think that this has to do with a hero status.

Vanlade is a hero for both EA Vanheim and Helheim, and the MA/LA nations, yet I never heard about a situation like this from games where Vanheim and Helheim played together. In the big Megagames with nations from all era, there would have been a lot of occurences like this if the code for national heroes would allow this situation.