On The Bakery, there's a new expanded tutorial (from this) that talks about how to check for duplicate records in your CakePHP model.

[Here's how to] validate a form field (such as a user name field), both in add and edit form and make sure that the selected user name does not already exist in the database [via a] function repeated only once (in app/app_model.php).

The example model they give defines an isUnique method that essentially runs an automatic check (a count() call) on the table to see if the given information exists. The example Model, View, and Controller are all given.