set record in tempdata

I have use this code set record in TempData .But Here CategoryId Multiple time for e.g 261 id 3 time but i want to add this id only one time like 261 one time

foreach (var customEntity in customEntityResult.Items)

{

var model = (BlogPostDataModel)customEntity.Model;

var blogPost = new BlogPostSummary();

blogPost.Title = customEntity.Title;

blogPost.ShortDescription = model.ShortDescription;

blogPost.ThumbnailImageAsset = images.GetOrDefault(model.ThumbnailImageAssetId);

blogPost.FullPath = customEntity.PageUrls.FirstOrDefault();

blogPost.PostDate = customEntity.CreateDate;

blogPost.CategoryId = model.CategoryIds.Distinct().SingleOrDefault();

blogPosts.Add(blogPost);

}

TempData["blogPostUrl"] = JsonConvert.SerializeObject(blogPosts);