Guard against duplicate keys for 4.0 toolset.
Guard against duplicate keys for 4.0 toolset.

AndyGerlicher/msbuild,mono/msbuild,mono/msbuild,sean-gilliam/msbuild,mono/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,cdmihai/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,mono/msbuild,chipitsine/msbuild,Microsoft/msbuild,chipitsine/msbuild,jeffkl/msbuild,cdmihai/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,cdmihai/msbuild,jeffkl/msbuild,jeffkl/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,chipitsine/msbuild