If one cell is the same as another

I am keeping an email list for my university and there are a few hundred emails on it. Due to the way this works, there are various people who have registered for it twice. And we don't want to send one person the same email multiple times. Since every month it automatically updates itself, it is quite annoying to have to manually look over a few hundred emails to see if any match.

I can sort them into alphabetical order, which makes it easy, but I was wondering if there were any short cuts out there? So I could identify if any cells in C match any other?

edit: Just to let you know what I do at the moment, is I make all the 'new' emails that come in every month bold. So when I sort it is easier to look to what has just been added to see if it has been replicated. This works alright but I'd like to know a more permanent and perfect solution.

Thanks in advance guys,
discomute