Color selection tool

[evigor]

Dear all,

I'm vectorizing images with the help of GIMP. I start with an image like this:

then with the color selection tool I select all the lines:




ad after I export the selection to Qgis, with an apposite tool. The problem is that the selection for each line  is "doubled": one for one side, one for the other. Here a zoom:

 

Well...is there the possibility for GIMP (maybe with a different tool) to select with just a "line selection", right in the center of the original lines?

Thank you