Hi Stefan, it means that there are multiple control points of the curve stacked up right on top of the same location.

That can be problematic because it degrades some of the mathematical properties of the curve at those spots, it's not a bad idea to reconstruct curves that have those in it before you try to build surfaces from them. If you end up with surfaces that have a bunch of control points stacked on on the same location (other than the pole point for a sphere like one), that can prevent intersections from working very well.

That particular curve looks like it's pretty bad with every single point maybe being a stacked up control point.

- Michael