I find a lot of mis-spellings by looking at a phone field or and address field.
Other than that there isn't a great way to handle 'similar' records like you described.  The computer is pretty black and white.  In this case Prez and Perez equal and either could be legitimate.

Yea you wouldn't want to base it on the name either, very likely to have two customers with the same name, If you had a Sergio Prez and went to add another person with the name Sergio Prez it wouldn't let you, even if they were two different people. The best way to do duplicates is with a field that is unique to a person,  that only one person could have at any one time. Like Scott said, phone is a good one, address is good if you account for apartment complexes having multiple people in one location (potential for two customers at one address), email is a good one.

Like the guys said, you need some other way to determine if they're duplicates. Names are ENTIRELY unreliable. Phone number. Email address. Social Security Number. Street address and ZIP. Anything but name.