Add automatic deletion of duplicates

[Stephen-Cronin]

Requested here: https://wordpress.org/support/topic/automatically-delete-duplicate-posts/

Need to consider the wp-cron approach via a setting as the main approach. Might be worth considering an auto function that runs for new posts and disallows duplicates if a match is found.

For all of these approaches, we'll need to handle redirects if the Redirection addon is present.

[Stephen-Cronin]

FYI, removed wp cli idea as that is a separate issue - #9