Feature Request: show that a link is already open in a tab

[hmijail]

It'd be useful to show to the user that the link they are hovering is currently open in a tab. I can imagine this could be done by changing how the link is displayed, or by changing the mouse cursor when hovering the link.

I proposed this feature to another add-on because, being related to the mouse cursor, I thought it might fit there better. But given that seemingly one difficulty is dealing with the logic of what is a duplicate, maybe it makes more sense here.