When I found out that my friend was starting to collect a 40k army I didn't want him to, because I'm already collecting.
What makes you not want him to share your hobby?
I just don't like not to be the best at something, and he might outdo me.
You are obviously proud of your collection, but it's not right to stop your friend from collecting too. Try thinking of this as an activity that the two of you can do together. I'm sure it would be fun.
I can't help how I feel. He broke my heart by trying to take over my hobby.
Hobbies are meant to be fun, and their more fun when you can share them with a friend. Instead of thinking about this like it's a competition, maybe you could try hunting for these collectibles together.