Add a project option "Allow only 1 instance" for run configurations

From "Run Configuration", it should be possible to tell QtCreator to kill an existing application before it starts up a new process of the same name. It might even be a nice global setting too.

PyCharm has a run config option: "allow only 1 instance" which is pretty much what i am describing.

This feature is especailly important for windows users who are attending Qt Training, because without this feature, a lot of old processes end up lying around and we get file locking issues when we try to rebuild.