An item with the same key has already been added

Question

I get this error whenever I submit the form also the action method is not being called because of this:

An item with the same key has already been added.

And the exception details:

[ArgumentException: An item with the same key has already been added.]
System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +52
System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +9382923 System.Linq.Enumerable.ToDictionary(IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer) +252
System.Linq.Enumerable.ToDictionary(IEnumerable`1 source, Func`2 keySelector, IEqualityComparer`1 comparer) +91
System.Web.Mvc.ModelBindingContext.get_PropertyMetadata() +228 System.Web.Mvc.DefaultModelBinder.BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor) +392
System.Web.Mvc.DefaultModelBinder.BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) +147
System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +98
System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +2504
System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +548
System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) +473
System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +181
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +830 System.Web.Mvc.Controller.ExecuteCore() +136 System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +111
System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +39
System.Web.Mvc.<>c__DisplayClass8.b__4() +65 System.Web.Mvc.Async.<>c__DisplayClass1.b__0() +44 System.Web.Mvc.Async.<>c__DisplayClass8`1.b__7(IAsyncResult _) +42 System.Web.Mvc.Async.WrappedAsyncResult`1.End() +141 System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +54
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +52
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +38
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8836913 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184

ViewPage

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/XYZ.Master"
    Inherits="System.Web.Mvc.ViewPage<XYZ.Models.Admin.AdminSegmentCommissionsModel>" %>
    <asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
        Create
    </asp:Content>
    <asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
        <% using (Html.BeginForm()) {%>    
            <div class="box3">
                <div class="userinfo">
                    <h3>Admin Segment Commissions</h3>
                </div>
                <div class="buttons-panel">
                    <ul>
                       <li>
                           <input type="submit" value="Save" class="save" />
                       </li>
                       <li>
                           <%:Html.ActionLink("Cancel", "Index", new { controller = "AdminSegmentCommissions" }, new { @class = "cancel" })%>
                           <%--<input type="button" value="Cancel" class="cancel" onclick="document.location.href='/AirlineLedgerTransactionReceiver/Index'" />--%>
                       </li>
                   </ul>
               </div>
           </div>
           <div class="row-1">
               <div class="form-box1 round-corner">
                   <div class="form-box1-row">
                       <div class="form-box1-row-content float-left">
                           <div>
                               <label>
                                   <%: Html.LabelFor(model => model.FromSegmentNumber) %></label>
                                   <%: Html.TextBoxFor(model => model.FromSegmentNumber) %>
                                   <%: Html.ValidationMessageFor(model => model.FromSegmentNumber) %>
                          </div>
                      </div>
                  </div>
              </div>
          </div>
      <%} %>

Answer 1

Most likely, you have model which contains the same property twice. Perhaps you are using new to hide the base property.

Solution is to override the property or use another name.

If you share your model, we would be able to elaborate more.