As far as I know, by default DSPCB 8 does not allow you to connect with a track two points that are already connected. You can change settings to allow you to do that leaving both connections, which is something that can be useful but a bit tricky. I would find interesting to add another option to allow the program to automatically delete old routing if you trace a new one connecting the same points. I find this very handy when I am designing complex PCB and I am modifying track routes to obtain the most optimal solution (at least the most optimal I can find).