This is the GTA On Mission Changer that can be used to test "on mission flag" related dupes in GTA 3, VC or SA. I decided to make the code publicly available so people can help improve it if they want. I will check the issues and pull requests every now and then and release a new version if needed.
The code could be cleaner but it was made rather quickly years ago when I was still fairly new to programming so I guess it's all right.
You can find the latest version on the Releases tab