Re: What if..
^You can find bigger and better projects as Mirch has suggested. Just be careful that your superiors don’t get the wrong idea that you’re not interested/couldn’t handled/or completely ignored the previous assignments that you had. Maybe clarify things before moving on to something new.
Or thank the person for their help and tell them that you’ll resume handling the projects in their entirety and that you don’t want to keep them from THEIR OWN assigned tasks (hint hint). I am confused as to how exactly the person would continue to work on your projects when you’ve returned are physically there at work…unless you’re giving them directions on the project or delegating. If he/she persists, then tell your boss that you’d like some confirmation/clarification as to who is doing the project.
If the person has made changes to your project that you don’t agree with, then do the project your own way…so that they can’t claim credit for it later on.