How do I seperate shared vertices when creating seams for UV unwrapping? I have a square mesh, I create a seam along the top edges (like a lid) and I UV unwrap the whole thing.

The UV mesh looks correct, but when I select an edge, the corner vertice is shared by an edge that is not part of the seam as well. So the UV mesh becomes selected in multiple places. 

How can I get a vertice on a seam to separate from a shared edge?

Does that makes sense?
Maybe switch the selection mode to Shared Location, it will take the seam into account: