I have 2 users in the system (PAUL & MIKE) and these 2 users have 1 lead each that has the same email. 

Paul has his lead John White with the email john@aol.com

Mike has his lead Carl Black with the email john@aol.com

When I go to see the contact details in the specific lead page, I see emails from both users the email sent from Paul to John and the emails sent from Mike to Carl.

Is it possible to avoid this kind of issue? Is it possible to avoid duplicates in the system that can lead to sharing issues like this?

Thanks