I never understood this. It's not as if there's some 'rule of war' that one side cannot use equipment based on designs of the other side. Where China clearly has based its designs on foreign kit (e.g. Z20, J-11/15/16, H-6K, Z-9, many others) - whether by fair means or foul- then they've saved themselves billions and billions of $$$ in design & development costs - what's not to like? In other cases it's clearly a matter or form following function - the way pretty much every current or planned 5th Gen fighter (except of course J-20) looks like something between F22 and F35 is because they're all trying to meet similar design requirements.