Compiler Error Message: CS0433: Type "System.Web.Helpers.Json" exists in both

I just checked a .NET project to work. However, when I run it, it gives me this compilation error:

Compiler Error Message: CS0433: Type "System.Web.Helpers.Json" exists as "c: \ Windows \ Microsoft.NET \ Framework \ v4.0.30319 \ Temporary ASP.NET Files \ root \ 2c3c3525 \ f643d2c6 \ assembly \ dl3 \ a363418d \ c830c6f0_9d86d001 \ System.Web.Helpers.DLL 'and' c: \ Windows \ Microsoft.NET \ assembly \ GAC_MSIL \ System.Web.Helpers \ v4.0_2.0.0.0__31bf3856ad364e35 \ System.Web. Helpers.dll

Line 57: db.SubmitChanges (); Line 58: Line 59: Context.Response.Write (Json.Encode (new {success = true})); Line 60: Line 61:}

I tried looking for a solution on the internet but it didn't help. Next steps:

In manually removing all content inside the TEMP file, adding batch = "false" to the compilation tag in the webconfig file

None of them helped ... Anyone have an idea?