Why in "Configure Data Source" when i choose "Parameter source :Control" all ControlIDs are duplicated?

I don't know it happen just to me or Others have the same situation too .

All the time when i bind a DataControl to an ObjectDataSource in "Configure Data Source" wizard of ObjectDataSource,开发者_JAVA技巧when i set parameter source as "Control" in "ControlId" DropdownList there are TWO Item of every control in the WebForm , while it make sense be one .

Configure Data Source http://sites.google.com/site/dindarbiz/_/rsrc/1262726225583/file-cabinet/ObjectDataSource.jpg

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Finally I found out it happen only when i use Master page , But i don't know why .