There is a solution here: How can I resolve the error "Net wire X has multiple names"?
However, JACK_IN_GND and JACK_OUT_GND will go to two different directions (one input cable and one output cable). And GNDs should be tied together.
Why is having two names so bad? Should I simply call it GND? And assuming since it's next to another wire (JACK_IN_SIGNAL and JACK_OUT_SIGNAL) that it is a GND signal for IN resp. OUT?