Last week, I saw some internal discussion about trying to locate the source of a duplicate object error on-premises. While an advanced administrator will be able to figure it out by looking at the Connector Spaces for connected directories, it's not necessarily obvious to a lot of people (especially if you're not experienced with our identity management products).
So, I had created a tool (simply named "Find duplicate addresses") that can be used to pinpoint a specific conflicting object. However, it didn't seem to be performing in really large environments (tens or hundreds of thousands of objects). I've tweaked it a little bit, and now it should work fine in larger environments (I've tested up to 100,000 objects in an Active Directory forest).
You can pick up the updated tool at https://gallery.technet.microsoft.com/Find-Duplicate-Values-in-6b012059.