After evaluating this list of modules, I went with Unique Field Ajax, which is quick to set up.
After enabling the module, edit the Unique title field per content type.
Modules
Allow Only One
View Allow Only One on Drupal.orgAllow Only One
Prevent duplicate content based on several rules (title, field values).
Existing Node Warning
View Existing Node Warning on Drupal.orgExisting Node Warning
Looks like this works on save.
Unique Field
View Unique field on Drupal.orgUnique field
Similar to Allow Only One.
Unique Field Ajax
View Unique Field AJAX on Drupal.orgUnique Field AJAX
Client side validation for unique field values.
Anti Duplicates
View Anti-Duplicates on Drupal.orgAnti-Duplicates
Shows similar content by title in the create content form.
Unique Alias Checker
View Unique Alias Checker on Drupal.orgUnique Alias Checker
Prevent duplicate aliases.
Field Validation
Has a unique field option.
Looks like 3.x uses Symfony UX Validation.
View Field Validation on Drupal.orgField Validation
Clientside Validation
View Clientside Validation on Drupal.orgClientside Validation
Check unique
View on Drupal.org #1494974Unique validation Status: Needs work