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  lt  c  DisplayClass8 b  4      65    System Web Mvc Async  lt  c  DisplayClass1 b  0      44    System Web Mvc Async  lt  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 amp  completedSynchronously     184   ViewPage   lt    Page Title    Language  C   MasterPageFile    Views Shared XYZ Master      Inherits  System Web Mvc ViewPage lt XYZ Models Admin AdminSegmentCommissionsModel gt     gt       lt asp Content ID  Content1  ContentPlaceHolderID  TitleContent  runat  server  gt          Create      lt  asp Content gt       lt asp Content ID  Content2  ContentPlaceHolderID  MainContent  runat  server  gt           lt   using  Html BeginForm       gt                   lt div class  box3  gt                   lt div class  userinfo  gt                       lt h3 gt Admin Segment Commissions lt  h3 gt                   lt  div gt                   lt div class  buttons-panel  gt                       lt ul gt                          lt li gt                              lt input type  submit  value  Save  class  save    gt                          lt  li gt                          lt li gt                              lt   Html ActionLink  Cancel    Index   new   controller    AdminSegmentCommissions     new    class    cancel      gt                              lt  -- lt input type  button  value  Cancel  class  cancel  onclick  document location href   AirlineLedgerTransactionReceiver Index     gt --  gt                          lt  li gt                      lt  ul gt                  lt  div gt              lt  div gt              lt div class  row-1  gt                  lt div class  form-box1 round-corner  gt                      lt div class  form-box1-row  gt                          lt div class  form-box1-row-content float-left  gt                              lt div gt                                  lt label gt                                      lt    Html LabelFor model   gt  model FromSegmentNumber    gt  lt  label gt                                      lt    Html TextBoxFor model   gt  model FromSegmentNumber    gt                                      lt    Html ValidationMessageFor model   gt  model FromSegmentNumber    gt                             lt  div gt                         lt  div gt                     lt  div gt                 lt  div gt             lt  div gt         lt      gt

User · Answer

I would like to add an answer that I do not see here. It is very related to the accepted answer, however, I did not have duplicated properties on my model, it was an issue with my Javascript.

I was doing some Ajax save where I was rebuilding the model to send back to the server. When I had first initialized the page I set my original model to a variable:

var currentModel = result.Data;

My result.Data has a property: result.Data.Items

So, some time later, I do some things and want to save, via Ajax. Part of the process is to grab an array from some side process and set it to my currentModel.Items property and send currentModel to the server.

In my Javascript, however, I did this, instead:

currentModel.items = getData();

I didn't catch it, but in Visual Studio, it will auto lower case the first letter for Javascript properties (it could be a ReSharper thing too). Then, I got the exact error posted by OP when I tried to save because currentModel now has currentModel.items AND currentModel.Items

A simple change from "items" to "Items" fixed the problem.

User · Answer

I hit this in MVC 5 and Visual Studio Express 2013  I had two properties with an IndexAttribute like below  Commenting out one of them and recompiling resulted in scaffolding the MVC 5 controller with views  using Entity Framework succeeding  Mysteriously  when I uncommented the attribute  recompiled  and tried again  the scaffolder ran just fine   Perhaps the underlying entity data model or  something  was cached corrupted  and removing and re-adding the IndexAttribute simply triggered a rebuild of that  something     Index IsUnique   true   public string Thing1   get  set      Index IsUnique   true   public string Thing2   get  set

User · Answer

In MVC 5 I found that temporarily commenting out references to an Entity Framework model  and recompiling the project side stepped this error when scaffolding  Once I finish scaffolding I uncomment the code   public Guid CreatedById   get  private set       Commented out so I can scaffold      public virtual UserBase CreatedBy   get  private set

User · Answer

I faced similar exception  Check if all columns has header names  from select query in database  with exactly matching property names in model class

User · Answer

My problem was that I had a  Url Action and I had sent through a value for the same property twice

User · Answer

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

User · Answer

I had the same issue   i was foreach looping over my object and adding the result into a Dictionary lt string  string gt  and i had a  Duplicate in the key from the database    foreach  var item in myObject                        myDictionary Add Convert ToString item x                                       item y                item x had a duplicate value

User · Answer

In my case  simply when you compile the code  it runs just fine  But calling one of a static field of a static class that has a dictionary like sample code has  throws the exception   Sample code  public static AClass       public static Dictionary lt string  string gt  ADict   new Dictionary lt string  string gt                            test   value1     test   value2                   Explanation ADict has  test  key added twice  So  when you call the static class  it throws the exception

User · Answer

I had the same problem and this is how I solved it  I had a duplicate property with the same name in my ViewModel  One Property was in BaseViewModel and another is in derived Model   public class BaseviewModel    public int UserId   get  set         public class Model   BaseViewModel         public int UserId   get  set         I changed that to  public class BaseviewModel     public int UserId   get  set        public class Model   BaseViewModel       public int User Id   get  set        Now it is working fine

User · Answer

I had the problem not in my C  model  but in the javascript object I was posting using AJAX   I m using Angular for binding and had a capitalized Notes field on the page while my C  object was expecting lower-case notes   A more descriptive error would sure be nice   C    class Post       public string notes   get  set        Angular Javascript    lt input ng-model  post Notes  type  text  gt

User · Answer

In my case the root of the problem was duplicate property name in the client json which only differed by case sensitivity

User · Answer

I had this issue on the DBContext  Got the error when I tried run an update-database in Package Manager console to add a migration   public virtual IDbSet Status   get  set     The problem was that the type and the name were the same  I changed it to   public virtual IDbSet Statuses   get  set

User · Answer

I found the answer It was because of the variables  Like int a and string a  there were two variables with the same name

User · Answer

I have had the same error  And after I have already thought my mind is broken  because I had rename almost all my models properties the solution was delete one reference on All Syncfusion Controls and add references to the individual controls of this controls   From Nuget

User · Answer

Here is what I did to find out the key that was being added twice  I downloaded the source code from http   referencesource microsoft com DotNetReferenceSource zip and setup VS to debug framework source  Opened up Dictionary cs in VS ran the project  once page loads  added a debug at the line ThrowHelper ThrowArgumentException ExceptionResource Argument AddingDuplicate   and was able to see the  key  value  I realized that in the JSON one letter of a variable was in upper case but in my model it was lowercase  I fixed the model and now the same code works

User · Answer

I had a similar problem and found that it was because I had a similarly named public property  that should have been private  that only differed in case   public string PropertyName  get set      actually set propertyName  get propertyName public string propertyName  get set     should have been  public string PropertyName  get set    private string propertyName  get set

User · Answer

I had 2 model properties like this  public int LinkId  get set   public int LinkID  get set     it is strange that it threw this error for these 2 haha

User · Answer

Another way to encounter this error is from a dataset with unnamed columns   The error is thrown when the dataset is serialized into JSON   This statement will result in error   select  column1   column2   Adding column names will prevent the error   select  column1 as col1   column2 as col2

User · Answer

I have had the same error but b c of diff reason  Using Entity framework  One more thing I need to add here before I share my code and solution  I had a break point on controller method but it was not breaking there  just throwing exception 500 internal server error   I was posting data from view to controller through ajax  http post   The model I was expecting as a parameter was a class  It was inherited with some other class   public class PurchaseOrder   CreateUpdateUserInfo               public PurchaseOrder                         this Purchase Order Items   new List lt clsItem gt                        public int purchase order id   get  set            public Nullable lt int gt  purchase quotation id   get  set            public int supplier id   get  set            public decimal flat discount   get  set            public decimal total   get  set            public decimal net payable   get  set            public bool is payment complete   get  set            public decimal sales tax   get  set            public DateTime CreatedOn   get  set            public int CreatorUserID   get  set            public DateTime UpdatedOn   get  set            public int UpdatorUserID   get  set            public bool IsDeleted   get  set            public List lt clsItem gt  Purchase Order Items   get  set            public class CreateUpdateUserInfo               public DateTime CreatedOn   get  set            public int CreatorUserID   get  set            public string CreatorUserName   get  set            public DateTime UpdatedOn   get  set            public int UpdatorUserID   get  set            public string UpdatorUserName   get  set            public bool IsDeleted   get  set            and in view                   var model                     supplier id   isNaN  scope supplierID     0   null    scope supplierID                  flat discount   0                  total    scope total                  net payable     scope total                  is payment complete    true                  sales tax 0                  Purchase Order Item   scope items                            var obj                     method   POST                   url   Purchase SaveOrder                   dataType   json                   data  JSON stringify model                   headers     Content-Type    application json                                var request    rootScope AjaxRequest obj               request then function  response                    var isError   response data MessageType     1                   rootScope bindToaster response data MessageType response data Message                        html  body   animate   scrollTop  0     slow                    if  isError                          scope supplierID  undefined                                   function  response                     rootScope bindToaster 2 response data                   console log response                     Simply removed duplicated fields from PurchaseOrder class and it worked like a charm

User · Answer

I have had the same error  When i check the code then i found that declare  GET  request in my angular  font-end  side and declare  POST  request in the ASP net  back-end  side  Set POST GET any one in both side  Then solved the error

User · Answer

I had same issue  It appeard to me after migrating to MVC 5 from MVC 3   I had custom editor template and had to use it like this before    Html EditorFor model   gt  model MyProperty   MyCustomTemplateName    MyPropertyField   this ViewData    To resolve issue I had to remove passing ViewData object  So at the end I had    Html EditorFor model   gt  model MyProperty   MyCustomTemplateName    MyPropertyField     Hope it helps

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