Sora front derailleur friction shifter
You may be able to adjust the friction. Adjusting the tension in the cable probably won't help.  

This is a bit of a known issue with some combinations:


https://www.bikeforums.net/bicycle-mechanics/1069395-shimano-rd-5701-hard-shift-friction-shifters.html
http://forums.roadbikereview.com/components-wrenching/shimano-friction-shifter-adjustment-183941.html


Unfortunately you're unlikely to be able to find a weaker return spring to fit.