Hello Elbino,

This is actually the expected behavior.  The way the tabs work in the dockmanager is similar to how they work in the Microsoft TabControl.  Since only one tab can be in view, inside the template for the tab control there is a content area.  When a tab becomes selected, the tab's content gets placed inside this content area which in turn places the content into the Visual Tree.  Once the content is added to the Visual Tree, the Loaded event will fire.  When a new tab is selected, the old content is removed from the content area and new content takes its place.  This means the old tab content is taken out of the Visual Tree.  This back and forth will cause the Loaded event to fire multiple times.

If you only want to handle code in the event once, you'll have to keep track of whether the content was loaded or not and execute the code accordingly.  For example: 

private bool _isLoaded = false;

private void ComboBox_Loaded(object sender, RoutedEventArgs e)
{
    if (!_isLoaded)
    {
        // execute load code.
        // ...
        
        _isLoaded = true;
    }
}