Mutexes are cleaned up automatically by the OS when a program quits, even if it quits abnormally.
Here’s an example project that implements as a custom subclass of the Application class called SingleInstanceApp. The SingleInstanceApp class behaves like a normal Application class except its OpenDocument event will be raised for both ordinary documents and also those sent over IPC from another instance.
You can copy the SingleInstanceApp class into your project and then change the App superclass from Application to SingleInstanceApp.