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
https://www.drupal.org/project/allow_only_one
Prevent duplicate content based on several rules (title, field values).
Existing Node Warning
https://www.drupal.org/project/existing_node_warning
Looks like this works on save.
Unique Field
https://www.drupal.org/project/unique_field
Similar to Allow Only One.
Unique Field Ajax
https://www.drupal.org/project/unique_field_ajax
Client side validation for unique field values.
Anti Duplicates
https://www.drupal.org/project/anti_duplicates
Shows similar content by title in the create content form.
Unique Alias Checker
https://www.drupal.org/project/unique_alias_checker
Prevent duplicate aliases.
Field Validation
Has a unique field option.
Looks like 3.x uses Symfony UX Validation.
https://www.drupal.org/project/field_validation
Clientside Validation
https://www.drupal.org/project/clientside_validation
Check unique
https://www.drupal.org/project/clientside_validation/issues/1494974