Linq在加载时获得更多数据按钮单击开始索引和结束索引

我正在使用webApi和linq从SQL获取数据，现在我的页面上有“加载更多”按钮，当我单击时，每次应该带20条记录，我正在使用具有开始索引和结束索引的表列ID是主键，当我单击“加载更多”按钮时，我将从列表视图中获取最后一个ID表示最后一个数据ID，我正在使用该ID具有我的起始索引，现在我的问题是当我单击加载更多时会带来20数据，但以前的数据已被删除，当我第一次单击“加载更多”按钮时，我需要获取20 + 20 = 40数据，如何实现此目的。

 //Linq//
 public dynamic getALlImage(int userID, int LastImageID)
 {
    //here LastImageId is my Listview Last data ImageID
    if  (LastImageID!=0)
     {
       startIndex= LastImageID+20;
       EndIndex=LastImageID-20;
     }
       else
     {
      startIndex= 20;
       EndIndex=0;  
     }

  GetAllListFields = (from user in dbContext.UserTable
                      join img in dbContext.ImageTable on user.userId equals img.userID into UserVoice
                      from Image in UserVoice.DefaultIfEmpty()
                      where (Image.imgId <= startIndex && Image.imgId >= EndIndex && Image.userID == userID && Image.IsStatusChecked == false)
                      orderby feeds.CreatedAt descending
                      select new
                      {
                           PostImage = Image == null ? 0 : Image.ImageID,
                          Image = usrMessae==null?0: Image.userMessage,
                          createdAt = feeds == null ? DateTime.UtcNow : feeds.CreatedAt,
                          UserId = userdata == null ? 0 : userdata.userId,
                          Name = userdata == null ? " " : userdata.Name,
                          username = userdata == null ? " " : userdata.userName,
                          Email = userdata == null ? " " : userdata.emailID,
                        }).Distinct().OrderByDescending(x => x.createdAt).Take(startIndex).ToList();
   } 


        //HTML//
<div data-role="view"> 
<div>
 <ul  data-role="listview"   data-bind="foreach:ImageList">
 <li>
 <div data-bind="text:userID"></div>
 <div data-bind="text:userName"></div>
 <div data-bind="text:UserImageSrc"></div>
 <div data-bind="text:ImageId></div>
 <div data-bind="text:PostImageSrc"></div>
 <div >
 <button data-bind="click:getLoadMoreImageData">Load More</button>
 </div>
 </li>
 </ul>
 </div>
 </div>

//Knockout Js
function UserImageViewModel()
        {
            self=this;
          var lastModelValue = (self.FeedPostdata()[self.FeedPostdata().length - 1]);
          var lastPostImageId = (lastModelValue.ImageId())

         //On Load More click  last Image ID      
         self.getLoadMoreImageData=function()
         {     
           $('#loading').show();
           var UserModel = { userID:userID,ImageId:lastPostImageId}
           jQuery.support.cors = true;
           $.ajax({
            type: "POST",
            dataType: "json",
            url: serverUrl + 'api/xxx/xxx',
            data: UserModel,
            success: function (data) {
            self.ImageList($.map(data, function (item) {
                    return new ImageModel(item);
            }));},
            complete: function () {
                $('#loading').hide();
            },
            error: function (error) {
                jsonValue = jQuery.parseJSON(error.responseText);} });}}


        // On Page Load  without last Image ID  
          self.getALlUSerImage=function()
         {

            $('#loading').show();
            var UserModel = { userID: userID}
            jQuery.support.cors = true;
            $.ajax({
            type: "POST",
            dataType: "json",
            url: serverUrl + 'api/xxx/xxx',
            data: UserModel,
            success: function (data) {
            self.ImageList($.map(data, function (item) {
                    return new ImageModel(item);
            }));},
            complete: function () {
                $('#loading').hide();
            },
            error: function (error) {
                jsonValue = jQuery.parseJSON(error.responseText);} });
                }}
                }


               //Model//
              function ImageModel(item)
              {
                self.userId=ko.observable(item.userId);
                self.userName=ko.observable(item.UserName);
                self.UserImageSrc = ko.computed(function () {
                return "data:" + self.UserImageSrcType() + ";base64," + self.UserImageSrcBase64();});   

                self.ImageID=ko.observable(item.ImageId)
                self.PostImageSrc = ko.computed(function () {
                return "data:" + self.PostImageType() + ";base64," +  self.PostImageBase64();});                      
              }

           $(document).ready(function () {
           ViewModel = new UserImageViewModel();
           ko.applyBindings(ViewModel);
           }