Every row is executed twice - visual-studio

Question

I have found a bug - and I haven't found any solution to this.
I have a code in ASP.NET Core (using VSPro 2019 16.5.0):
public IActionResult CreateSubGroup(MyClass model, string returnUrl = null)
{
if (ModelState.CreateMyClassValidation())
{
if (!db.MyClass.Where(x => x.Title == model.Title).Any())
{
ViewData["ReturnUrl"] = returnUrl;
var code = new MyClass { Title = model.Title, IdGroup = model.IdGroup, GroupCode = model.GroupCode};
db.MyClass.Add(code);
var result = db.SaveChanges();
if (result > 0)//if there was no issue (at least one row was changed)
{
this.AddNotification(MessagesHandler.Success, $"Item\"{model.Title}\" was successfully created.");
}
else
{
this.AddNotification(MessagesHandler.Error, $"Item \"{model.Title}\" cannot be created.");
}
}
else
{
this.AddNotification(MessagesHandler.Error, $"Item \"{model.Title}\" already exists.");
}
}
else
{
this.AddNotification(MessagesHandler.Error, $"ErrorMessage.");
}
return RedirectToLocal(returnUrl);
}
Creating of new Item always crashes with unique code exception from DB - During debuging I have found, that every row is executed twice (and I don't know why??) - so also the row db.SaveChanges() is executed twice and that's why I got this exception.
Second bad thing is, that not even the first attempt to save database is not executed (= new Item is not created in DB).
Have you seen this error?
EDIT:
I have found, that it happens only when data are posted from view with JS/AJAX (from modal window)
Here is the code for sending data:
<div class="modal fade" id="ModalWindow" tabindex="-1" role="dialog" aria-labelledby="ModalForm" aria-hidden="true">
<div class="modal-dialog modal-dialog-centered" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title"></h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<form id="ModalForm" action="" method="post" class="validator">
<div class="modal-body">
<div class="form-group">
<label asp-for="Id"></label>
<input class="form-control" asp-for="Id" value="" readonly data-val="false">
<div class="form-text text-muted small">ID cannot be changed!</div>
</div>
<div class="form-group">
<label asp-for="Title"></label>
<input class="form-control mlfb-create" asp-for="Title" placeholder="Title" value="" autofocus tabindex="#(++tabindex)">
<span class="text-danger small" asp-validation-for="Title"></span>
</div>
<div class="form-group">
<label asp-for="IdGroup"></label>
<select class="selectpicker form-control" asp-for="IdGroup" data-live-search="true" data-style="btn-info" tabindex="#(++tabindex)">
#if (data?.GroupData != null)
{
#foreach (var item in data?.GroupData)
{
<option value="#(item.Id)">#item.Title</option>
}
}
</select>
</div>
<div class="form-group">
<label asp-for="GroupCode"></label>
<input class="form-control mlfb-create" asp-for="GroupCode" placeholder="Title" value="" autofocus tabindex="#(++tabindex)">
<span class="text-danger small" asp-validation-for="GroupCode"></span>
</div>
</div>
<div class="text-center modal-footer">
<button type="submit" class="btn btn-success _modal-buttton-save" tabindex="#(++tabindex)"><i class="fas fa-check mr-2"></i><span>Save</span></button>
<button type="reset" class="btn btn-secondary" data-dismiss="modal"><i class="fas fa-times mr-2"></i>Cancel</button>
</div>
</form>
</div>
</div>
</div>
</div>
</div>
#section scripts {
<script>
$(function () {
"use strict";
$(document).on('click', '._createSubFormButton', function () {
$('#ModalWindow').modal('show');
$('.modal-title').text('Creating of subgroup');
$('.modal-buttton-save span').text('Create');
$('#ModalForm').attr('action', '/MyCode/CreateSubGroup/?returnurl=' + window.location.href);
});
// Edit form
$(document).on('click', 'tr ._editSubFormButton', function () {
$('#ModalWindow').modal('show');
var $tr = $(this).closest('tr');
var Id = $tr.find('._Id').text();
var Title = $tr.find('._Title').text();
var IdGroup = $tr.find('._IdGroup').text();
var GroupCode = $tr.find('._GroupCode').text();
$('.modal-title').text('Editing of subgroup');
$('#ModalForm').attr('action', '/MyCode/EditSubGroup/' + Id + '?returnurl=' + window.location.href);
$('#Id').val(Id);
$('#Title').val(Title);
$('#GroupCode').val(GroupCode);
});
// form validation reset during closing modal form
$('#ModalWindow').on('hidden.bs.modal', function () {
$(this).find('form').trigger('reset');
$('#IdGroup').load();
$('.form-group .is-invalid').each(function () { $(this).removeClass('is-invalid'); });
$('.form-group .is-valid').each(function () { $(this).removeClass('is-valid'); });
$('.form-text.text-danger').each(function () { $(this).removeClass('text-danger'); });
$('.form-text.text-success').each(function () { $(this).removeClass('text-success'); });
$('.invalid-feedback').each(function () { $(this).remove(); });
});
$(document).on('submit', '#ModalForm', function (e) {
var form = $('#ModalForm');
if (form.valid()) {
console.log(form.serializeArray());
$.ajax({
url: form.attr("action"),
type: form.attr("method"),
data: form.serializeArray()
}).done(function () {
console.log('done');
$tr.find('._Number').text();
var $tr = $(this).closest('tr');
})
.fail(function () {
console.log('fail');
});
$('#ModalWindow').modal('hide');
}
});
error I got:

Answer 1

Have you tried debugging this code? Debugging with setting breakpoints and stepping through the code would help you find what is wrong with this code.