First of all, your demo site doesn't allow users to post ads, as they need admin validation.
Secondly, from my understanding, the Spam Solution Plugin Duplicate feature, checks a newly posted ad, against other ads from the same email, to see if it's a duplicate.
When a duplicate is found, the plugin gives you 2 options: Mark listing as spam and Delete listing.
For me, these 2 options don't work.
When you post a duplicate listing, it gets checked and marked with 100% similarity in the Refresh listings and show duplicates section, but it doesn't automatically get marked as spam, or deleted.
So in the end, the duplicate appears on the main page...