Listing, grouping, and counting duplicate email addresses in the Marketo UI

I put myself on the “no code needed” side of a friendly Community debate the other day, so this is a chance to prove I'm not always shoving people toward Velocity or JavaScript.

The requirement is a single report with:

1.  the list of email addresses linked to more than one lead
2.  the count of how many addresses are duplicated (i.e. simple row count of the above, without having to export + count separately)
3.  the count of how many times each address appears (how many potential dupes there are for each email)

You can get this done with a Lead Performance report and a nested Smart List.

First, the Smart List, which simply uses the Duplicate Fields filter:

Next, the Lead Performance report, which groups by Email Address:

And filters by Member of Smart List, nesting the Smart List above:

And there you have it:

In case it's not clear what's what in the output, here's the screenshot with some callouts:

You can add even more visibility by creating other Smart Lists with mutually exclusive filters, like Lead was Created with a [Source Type is] constraint:

Then add those to the report as custom columns:

And you've got more background on your dupes:

That's it!