It's common to attribute ownership to ideas when you work in teams. First, it's just natural to refer to them as "Jess' idea" or "Alex's idea", it makes it easy to identify them. And secondly, there is something about our egos and the way we feel about ideas we create. We tend to protect and try to make them win.
It's a good thing to fight for ideas we believe in, altho it's useless if we fight for them just because they are ours. When you work in teams to create solutions to a problem, it should never be a personal competition to see who contributed to the winning idea; it's about working together towards the same goal.
Idea ownership makes it difficult to "kill your darlings". It will be harder to let that idea if we feel attached to it because it's ours.
For avoiding this problem, we suggest you follow this tips and try to implement this mindset in your team slowly.
Name ideas anonymously
Never refer to a concept as "my idea" or "Charles' idea". Put names to it like "the social media idea", or "the poster idea". De-attach them from individuals.
Use methods that facilitate teamwork. There are tonnes of exercises that propose group-thinking. We have a good example here.
Reward the team, not individuals
Always compliment the collective effort. Talk as a team and solve problems together. Also, celebrate together!