Specified library or project already referenced
The Add References dialog box displays referenced libraries andprojects. This error has the following cause and solution:
You chose the Browse button in the Add References dialog box, then selected atype library or project that was already listed in the Available References section of the Add References dialog box.
To make a project or type library part of your project, display the References dialog box and make sure the type library or project name is checked.
For additional information, select the item in question and press F1 (in Windows) or HELP (on the Macintosh).