Adding autocomplete to title field of node form
Avoid a custom implementation and use the Unique Field Module. Once this module is enabled, it adds a 'Unique field settings' section to each content type's administration page for specifying which fields (Including node title) for that content type must be unique.The administrator may choose whether the fields must be unique among all other nodes, among nodes of the same content type, or of the same language. You can also set it to display the nodes that match the title.