title: The imported type is defined multiple times - only under xbuild on Mono 2.10.1
body: <p>Having an issue where compiling some code that makes use of <code>HttpContextBase</code> is throwing the compile error:</p>
  
  <pre><code>UserService.cs(13,26): error CS0433: The imported type `System.Web.HttpContextBase' is defined multiple times
  </code></pre>
  
  <p>The offending line is:</p>
  
  <pre><code>private readonly HttpContextBase httpContext;
  </code></pre>
  
  <p>If I take that line of code out, or comment it, the error goes away and the assembly compiles.</p>
  
  <p>In another project (the test project for this one) the same error occurs when trying to declare a <code>HttpContextBase</code> to pass to the constructor of this object.  So it appears to be a cross-project issue.</p>
  
  <p>Is this a project file issue? (Can't find more than one reference to <code>System.Web</code> in there).  It was compiling before I made some changes to the <code>AfterBuild</code> step, so I'm concerned I've messed something up in the csproj file.</p>
  
  <p>This is building with Mono 2.10.1, using monodevelop trunk build set to use xbuild to compile.</p>
  
  <p>Compiling using the default Monodevelop build behaviour does not produce this error.</p>