You'll want to make sure that your criteria are very specific - exact matches on Name (Or first name match on first name and Last Name exact match), Exact matches on Email, and throw in Account Name criteria, too. Those are the three things that yield the best results for me.
Perhaps if you share the criteria you're currently using, we can help you narrow it down.
Using email address is the best way to start finding those easier duplicates. Showing the account name along with contact information also helps so you can see if you have the duplicates on different accounts. Then have to figure out what is the most recent information for your merge.
