Should I prevent registration with a duplicate email address when the first one is not verified yet

I have a system that requires unique email addresses for registering and it verify that email using a verification link that we send to that address. In a situation like one person register with an email and waiting for it to click on the verification link, should I prevent other users to choose that email? What happened if I preventing them to choose that email and the older person never verifies that email? That email address would be not accessible to everyone and yet not in the system as well. What is the best approach is for this situation?