Can you please provide us a simple project to demonstrate the issue?

Also, I would like to know what is the version of the controls you are using.

I've created a XAML declared TabControl test and manage to reproduce the issue. The Docking control inherits from our TabControl thus inherits the drop down content features. When TabItems are declared in xaml and the header is identical with another tab item header TabControl will select the first occurrence when selection is performed from the drop down content. I've created a feature request item in our public issue tracking system. You could vote and track the issue via following link: http://www.telerik.com/support/pits.aspx#/public/silverlight/9230 

You could use databinding and declare the items in a view-model object. This way TabControl should be able to identify the objects correctly and perform a properly indexed selection.

Please let us know if you need further assistance.