An attempt was made to load a second system assembly {0}'. FxCop does not currently allow this. If a later version of the ...

An attempt was made to load a second system assembly ‘{0}'. FxCop does not currently allow this. If a later version of the same framework is required (version 4.0 vs. version 2.0) then try passing the /platform switch to FxCop to force it to use the later version.