Extend a line by adding a point at the end of it - QGIS 2.8

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I'm using QGIS to modify a line. I need to add a point on one end of this line, and then move this new point further away, leaving the original end point in the same place.

I can manage to create a new point at the end of the line by:

• entering Settings > Snapping options
• setting "Snap to" "To Vertex"
• setting a "Tolerance" that's > 0
• selecting the layer and clicking "Toggle Editing"
• selecting the "Node Tool"
• double clicking on the end of the line

And here's the problem. When I try to move the point I just created, I move the old line end instead.

Here's what happens, in pictures.

I want the old line end to stay there, untouched (same coordinates as before), while I want the new point I added to be moved. Instead, the opposite is happening.



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