‘Newtonsoft.Json…’ exists in both ‘Blend\Newtonsoft.Json.dll’ and ‘Solution\packages\…\

In your csproj file you will notice that there are 2 entries for Newtonsoft.Json. Remove the following entry:

<Reference Include="Newtonsoft.Json">
    <HintPath>..\packages\Newtonsoft.Json.5.0.6\lib\net45\Newtonsoft.Json.dll</HintPath>
</Reference>