A very common issue we deal with could be helped with a duplicate detection rule if Users and Queue entities were available in duplicate detection.

Customers often have queues set up for group email addresses e.g. customer.support@, technical.support@, etc. Large organizations with multiple business units can have dozens of these for different product lines, regions, etc.
So you set these up, go live and then you find later on that users are creating CRM contacts with these email addresses (they think they need that in order to send an email to them) without realizing these email addresses already exist on the queue records and that those can be used for emailing. I know we have auto-resolution that would populate the queue record if you type the email address in the TO field on the email and that trained users should know better than creating new contacts for this, however this is still happening in real life... happening a lot...
A similar scenario is constantly encountered when users create Contacts for existing CRM users, also for the purpose of emailing.

If we were able to define duplicate detection rules for Contact against the Queue and User entities, we would at least be able to prompt users that they are about to create something that is not necessary.
Unfortunately Queue and User entities are not available in duplicate detection, it would be great if they could be made available.

This was tested on CRM Online 2016 8.1.0.361.