Gmail now has a lab feature that inconspicuously warns you when you address an email to a first name that occurs more than once in your often-used contacts.
More information can be found on the official Gmail Blog.
Note: I had to test this out a few times before it would work. The key to testing it is to create a new email addressed to 3 people that you often email together. For one of them, choose the wrong name (same first name but different last name). Then, you should see the message.