Came across this in recompiling the POSTracker samples with the 4.3 binaries but the problem is due to having installed the ASP.Net MVC 4 preview (with VS2010 in my case).
Says ModelClientValidationRule is defined in two assemblies.
Found fix via http://stackoverflow.com/questions/7510484/modelclientvalidationrule-conflict
Essentially right mouse click the project, select "Add Deployable Dependencies" and pick MVC. It adds a _bin_deployableAssemblies folder to the project with the required dll's and then compiles ok.
Thank you Richard, same problem here, apparently due to it being so out of date with MVC 4 in vs2012, however apparently there is a simpler solution which is to remove the reference to System.Web.WebPages in the MVC3UI project.
Update: though now I see that the project loads the home page but none of the links that load business objects work so perhaps this was not the solution or it's just not compatible with vs2012 and mvc4.
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