How to select a specific vertex in UV unwrapping? I'm trying to UV unwrap one of my objects.

My problem is that I can't select a vertex in the UV window when it is at the edge of a Cut (Marked seam). Instead, it automatically selects the vertex on the other side of the cut.

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The red arrow indicates the vertex I selected. the blue circle indicates the vertex that gets selected.

Can someone tell me what the problem is here?
Just after I posted this question, I found the answer myself. 

I had this Button enabled


"Keep UV and edit mode mesh in sync"