There is a new settings added into App_Config\Sitecore.config file with Sitecore 8.1 rev. 160302 (Update-2), which relates to Duplicate item names.
AllowDuplicateItemNamesOnSameLevel Setting
As this setting name implies, you can enable/disable duplicate item names on same level of Sitecore content tree.
In earlier versions of Sitecore (earlier than 8.1 Update-2), it was possible to create duplicate item names in same level and there were no setting to disable that. On earlier Sitecore versions, you had to implement custom rules/custom coding to restrict duplicate item names.
<setting name="AllowDuplicateItemNamesOnSameLevel" value="true" />
With Sitecore 8.1 Update-2, you can disable duplicate item names by making the "AllowDuplicateItemNamesOnSameLevel" setting to 'false'
Description of the setting can be found below
<!-- ALLOW DUPLICATE ITEM NAMES ON THE SAME LEVEL
Specifies whether the duplicate item names are allowed on the same level in the content tree.
Default value: true
-->
Happy Sitecore!!