> encourages people to tinker with the code, but not to clone a copy and sell it. What I usually see happening is that they release two versions. One under GPL

> encourages people to tinker with the code, but not to clone a copy and sell it. What I usually see happening is that they release two versions. One under GPLv3, MIT or whatever very permissive licence and a Pro version with many advanced features with a proprietary licence. If you want to prevent forking and copying then I either suggest to keep it closed source or make sure that you maintain the best version of the app so users will have no motivation to switch to a fork and be loyal to the original creator.