Error occurred in deployment step 'Add Solution': Error: Cannot add the specified assembly to the global assembly cache: XXX.dll

Este erro ocorre no VS (Visual Studio) ao tentarmos fazer Deploy.

Error occurred in deployment step 'Add Solution': Error: Cannot add the specified assembly to the global assembly cache: XXX.dll

Como resolver:

1) Vá ao GAC (C:\windows\assembly) e remova a DLL manualmente. [remove DLL manually from GAC]
2) Realize deploy novamente [do deploy again!]