pleas help guys, my custom model binder which has been working perfectly has starting giving me errors

details below

An item with the same key has already been added. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: An item with the same key has already been added.

Source Error:

Line 31: { Line 32: string key = bindingContext.ModelName; Line 33: var doc = base.BindModel(controllerContext, bindingContext) as Document; Line 34: Line 35: // DoBasicValidation(bindingContext, doc);

Source File: C:\Users\Bich Vu\Documents\Visual Studio 2008\Projects\PitchPortal\PitchPortal.Web\Binders\DocumentModelBinder.cs Line: 33

Stack Trace:

[ArgumentException: An item with the same key has already been added.] System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +51 System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add) +7462172 System.Linq.Enumerable.ToDictionary(IEnumerable1 source, Func2 keySelector, Func2 elementSelector, IEqualityComparer1 comparer) +270 System.Linq.Enumerable.ToDictionary(IEnumerable1 source, Func2 keySelector, IEqualityComparer1 comparer) +102 System.Web.Mvc.ModelBindingContext.get_PropertyMetadata() +157 System.Web.Mvc.DefaultModelBinder.BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor) +158 System.Web.Mvc.DefaultModelBinder.BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) +90 System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +50 System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +1048 System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +280 PitchPortal.Web.Binders.documentModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) in C:\Users\Bich Vu\Documents\Visual Studio 2008\Projects\PitchPortal\PitchPortal.Web\Binders\DocumentModelBinder.cs:33 System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) +257 System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +109 System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +314 System.Web.Mvc.Controller.ExecuteCore() +105 System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +39 System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +7 System.Web.Mvc.<>c_DisplayClass8.b_4() +34 System.Web.Mvc.Async.<>c_DisplayClass1.b_0() +21 System.Web.Mvc.Async.<>c_DisplayClass81.b_7(IAsyncResult _) +12 System.Web.Mvc.Async.WrappedAsyncResult1.End() +59 System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +44 System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +7 System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8677678 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

any ideas guys? Thanks