Thanks, these are great tips! Let me add: Also use business-oriented, client-centric arguments - in speaking to your boss, you also want to stress the advantage to your client in reeling in the overenthusiastic Emily. Tell your boss your client would love it if they had a single point of contact in your business, and the project would work more efficiently for the client if the project manager (you) will always be in the loop. (You might even want to say something like this to the client as well, if you have a very strong relationship with them. If not, don't involve the client in internal politics!); 

It would also be more advantageous to your business to not have two heads working on one project. Say something like "Hey, Emily is so amazing and has such great ideas, she should really work on a project of her own! It's a waste of resources to have her on this project when I already have this very well in hand."

Your manager may well have your own career development close to their heart, but it would be harder for them to ignore advantages to the business and the client.