Error: Element is already the child of another element

[ShaningYu]

Received such an error as below

Error 1 Element is already the child of another element.   C:\...\App.xaml 11 17

<Application  
    x:Class="ArraProjects.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Application.Resources>
        <ResourceDictionary>
            <!-- Add all resource dictionaries -->
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Assets/Styles.xaml"/>
                <ResourceDictionary Source="Assets/ResApp.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

I searched around but got no good answer.  How can this bug be fixed?  Thanks in advance if you can share your experience.

[JenniferNery · Accepted solution]

This blog might help: http://whydoidoit.com/2010/08/30/debug-xaml-element-is-already-the-child-of-another-element/