The following IOException is thrown when I call Save on my AssemblyBuilderInstance.

The process cannot access the file because it is being used by another process. (Exception from HRESULT: 0x80070020)

If I delete the .dll being saved it works fine but then reoccurs if the Assembly exists before saving!

The AssemblyBuilder instance is being instantiated as follows
if (_assemblyBuilder == null)
{
AssemblyName assemblyName = new AssemblyName();
assemblyName.Name = "Compiled";
_assemblyBuilder = System.Threading.Thread.GetDomain().DefineDynamicAssembly(
assemblyName, AssemblyBuilderAccess.Save);

}
if (_module == null)
_module = _assemblyBuilder.DefineDynamicModule("Compiled.dll", true);