The unique property datatype allows you to make a property value on a node unique. A good example would be to make Page title unique for SEO purposes. To use this data type you need to create a new data type based on unique property. Decide on field alias you will be using and pass that into the field alias value of the datatype eg pageTitle.
Next add a property on your document type based on that datatype with field alias same as alias entered in the datatype definition eg pageTitle.
When a node is saved then for the current node the page title value is searched for in the xml cache of published nodes, if a duplicate value is found then the save event is cancelled and user is notified that the current page title value conflicts with the following existing node/s.