So I recently learnt Duplicate detection had been removed from CRM 2013. I was thinking about this and I guess you could add functionality to check this with a plugin yourself but this would of course mean you have to store configs and etc code and testing, which is quite annoying for functionality that use to be out of the box.
Then I saw that someone else has already created a solution to do it, yippeeee
It’s called CRM 2013 Duplicate Detection and is on codeplex, click the link to have a look
Provides the ability to show potential duplicate records on the client side when creating or updating records.
Uses the native duplicate detection rules
Works on standard and Quick Create Forms*
Not available for the tablet client
Configuration allows preventing save when potential duplicates exist