Microsoft decided to remove the Duplicate Detection feature in 2013 version. Why? Not sure.
The following article describes how to restore this functionality.

1. Download SDK for Dynamics CRM 2013 using the following link – http://www.microsoft.com/en-us/download/details.aspx?id=40321

2. Unpack it to your hard drive and open SampleCodeJSDuplicateDetection.

3. Import the solution DuplicateDetectionSample_1_0_0_0_managed.zip to your CRM:

4. Open the entity form where you want to activate feature (for example contact) and the add OnSave event handler, as it is shown in the following screenshots:

Here are the parameters that have to be passed to Onsave event handler:

false,
"contact", 
"contactid", 
["fullname", "firstname", "lastname","emailaddress1"],
"sample_/duplicateDetection/ShowDuplicateContacts.html",
"dialogHeight: 250px; dialogWidth: 600px; resizable: yes; status: no;"

5. Save the form and publish changes.

6. Open the new contact form and try to create a contact that would be a potential duplicate. If you’ve done everything right, you will see the following popup:

The full documentation and installation instructions are located in SampleCodeJSDuplicateDetection folder in the document ReadMe.docx.