You can set up a duplicate detection rule on two different entities. This allows you to compare the Email attribute of Contact against both the Email and Email 2 attributes of Account.

However, if you change the duplicate detection rule to run on one entity, you are no longer allowed to specify a second field...

This means you can only compare the Email attribute of a Contact to the Email attribute of another Contact. You cannot compare the Email attribute to the Email 2 attribute of a Contact.

I would like the ability to specify both a source and target attribute, even when dealing with only one entity in a duplicate detection rule.