I had a similar situation come up, and it was right around "ask for raise" time. Totally out of character for me, I decided to calmly but firmly stake my claim on the project I'd already started that another co worker was starting to take over
(scheduling meetings, sending emails to the team, etc).
I swear I was shaking the whoooole time, but in the end my co worker understood where I was coming from, that while I appreciated their enthusiasm and help with the project, I was going to handle it.
And I did. It kind of made me realize that sometimes people don't necessarily realize they're being a ball hog and sometimes you just have to have a calm, professional, clearing of the air.